Make a puree first by putting the blackberries in a saucepan with the water, bringing to the boil, then reducing to a simmer for 10-15 mins.
Pass through a nylon sieve - draining initially, then forcing the fruit pulp through until it's dry and exhausted. Discard the pulp and put the puree (I got 350ml) in the top of a double boiler, or in a pyrex bowl over a saucepan of simmering water if that's what you have.
Add the caster sugar, stirring to melt and slowly whisk in the beaten eggs.
Keep the curd on the heat whilst you continue to stir. Eventually it will thicken, although this may take upwards of 25 minutes.