Luocha

The most potent healer in the game as of this writing, Luocha is also the one of the rarest due to being a limited 5* character.

Luocha’s Skill allows him to heal allies when their HP drops to 50% or lower without using SP.  When paired with Blade and his self-damaging abilities, this effect will be often triggered, automatically restoring Blade's HP. This will also allow Blade to start the fight with his Ultimate, and then be healed all the way back up. 

The field casted by Luocha's Talent will also restore Blade's HP automatically whenever he attacks inside this field, which will provide an additional source of healing.

When Blade and Luocha are paired together, team survival is practically guaranteed. In situations where there is a great need for healing, such as fighting against the strongest enemies, or when paired with Bronya with Slowya/Hyperslowya builds, Luocha will be your best choice for team sustain.

Luocha is also a good option in teams with high SP requirements, due to his own low SP needs. And in Simulated Universe modes (especially Abundance path) he will pull ahead any other healers due to the many gained buffs caused by his ample healing.

Although he may be an expensive option, Luocha is nonetheless still a great investment for Blade teams.

Lynx

Lynx presents great synergy with Blade due to being a sustain option that also provides useful buffs.

Lynx’s Skill heals an ally and grants them an HP% buff based on her Max HP, which Blade welcomes not only due to the healing aspect, but because this will also increase his damage as he greatly benefits from HP% buffs.  

On top of that, if the ally is a Preservation or Destruction character, Lynx's Skill also increases their chance to be hit. This will help Blade gain stacks for his Talent. As an added bonus, being hit after Lynx's  skill is casted on an ally also helps them recover energy.

Lynx's Ultimate will heal the full party, and cleanse one debuff from all party members.

Additionally, she provides further healing through her Talent, which causes the target of her Skill or Ultimate to receive continuous healing for 3 turns.

Although Lynx's healing potency is inferior to Luocha's, she may still be preferred over him in certain scenarios, such as where the need for healing isn’t that high and the damage increase caused by her useful buffs may be more appreciated. When great amounts of healing are needed, however, Luocha may be considered a better choice. Your other teammates' SP usage is also important to have into account when choosing which of the two may be better to use in your team.

Lynx is a great sustain choice for Blade with added support capabilities that can also be cheaper than other alternatives due to her status as a 4 star. And even though she reaches her max support potential at E6, she’s totally viable at E0, which makes pairing Blade and Lynx very affordable.

Huohuo

As the second strongest healer in the game after Luocha, Huohuo has amazing healing and cleansing capabilities that will easily guarantee your team's survival.

Huohuo's Skill dispels 1 debuff from an ally and heals them, as well as healing adjacent allies at the same time.

Her Talent also allows her to heal her teammates every time someone gets a turn, as long as she has enough passive stacks, which she will obtain by using her Skill. 

Huohuo's Ultimate regenerates energy for the whole team (except herself) and will buff the team's ATK. 

Having to maintain her passive stacks by using her Skill often makes Huohuo a bit more SP intensive than other sustain options such as Luocha, but she provides great utility in exchange. Her Ultimate's buff is not that useful for Blade either (although not completely useless), as he does not benefit from attack buffs that much. 

For these reasons, Huohuo may be less of an attractive option in comparison to Luocha or Lynx. However, she is still a perfectly capable healer that will work well in Blade teams, and if you don't have the previous two options but have Huohuo, she may be your best choice.

Bailu

Bailu is another good sustain option, and more accessible than other 5 star characters like Luocha or Huohuo, since she is available as a Standard 5 star character.

Bailu's skill heals a single ally, and then heals random allies 2 times. Her Ultimate heals the party and also applies Invigoration.

Her Talent will restore extra HP to Invigorated allies, as well as prevent an ally from dying when their HP is fully depleted (although only once per battle) which extra contributes to her sustain capabilities. In her Talent Blade can also find an useful max HP buff granted when overhealing, as well as damage reduction when Invigorated.

Although Bailu's healing power is inferior to Luocha's or Huohuo's, she can still contribute enough healing in most situations, as well as provide some appreciated damage reduction. She will be useful as well by providing HP buffs to boost Blade's damage. Although there are other healers better than her to pair with Blade, Bailu is still competent and can be used in Blade teams effectively.

Natasha

Natasha is the most widely available healer due to being a 4 star character that is given for free, and in some cases the only option some players may have for the sustain slot, especially for those new to the game.

Her Skill removes 1 debuff from an ally and heals them once, and then heals them again at the beginning of the next two turns. Her Ultimate also heals the whole party. Her Talent will boost her healing power.

Of all the healers in the game, Natasha is by far the weakest in terms of healing potency, which makes a Blade team with her a bit more dangerous. Nevertheless, she is viable and does the job well enough.

Jingliu

While there are a few other DPS characters that can work well in a team along with Blade, Jingliu is the only one who has any relevant synergy with him so far.

Thanks to her Talent, Jingliu gains 1 stack of Syzygy when using her Skill or Ultimate. When Jingliu has accumulated 2 of these stacks, she enters the Spectral Transmigration state. When in this state, her attacks will drain HP from her allies. This HP drain will lower Blade's HP, and trigger his follow-up attacks more often.

Although not a direct synergy, it is worth noting that both Blade and Jingliu are compatible in several ways.  They both benefit from HP buffers and are usually in need of strong heals, which makes their individual synergies with other characters align, and choosing their teammates an easier task. Blade and Jingliu are both SP efficient characters as well, which helps by making their choice or teammates more flexible, and eliminates the common SP management issues other Double DPS comps may usually face.

Jingliu is a great damage dealer in her own right, and currently your best choice if you are looking for a second DPS unit which to pair with your Blade to create a powerful dual carry team.

Preservation Characters

Most preservation characters don't pair well with Blade, for several reasons. 

Blade prefers to take damage so his Talent can be activated and gain stacks from it. But Preservation characters have a higher Taunt Value, so they are more prone to getting hit than Blade is. 

Most of them also offer shields, and shields prevent Blade from getting hit and gaining stacks if they are not broken when hit.

However this does not mean that no Preservation character can be paired with Blade (Fu Xuan can be an option as explained above), but in general, most of them aren’t recommended.

Clara

As a unit, Clara is centered around her counter, and as such, she wants to get as many hits on herself as possible. Blade is not dissimilar to her in this regard, so there's a conflict between the two of them when on the same team that leads to neither one getting their maximum DPS output.