The name is derived from the Old English pre 7th century words 'bloec' meaning black, and 'loece' meaning a boggy stream.

Bloecloece became Blackleech (and other variations such as Blakelache), which became Blackledge.

"Recorded as Blackledge and Blacklidge, this very unusual surname is English.
It is locational and originates from a now "lost" medieval village [not lost - JB] thought to have been near to the town of Leyland in the county of Lancashire, and called Black Leache.

600AD: The derivation is from the Olde English pre-7th century word
'bloec' meaning 'black' plus 'loece', a boggy stream.

1150: Earliest written recording of the name found in PRESTON dated 1150 (less than 100 years after 1066). 

1207: Another written record found in LIVERPOOL dated 1207.

1332: An alternative record states:
"The first recorded spelling of the family name is shown to be that of John del Blakelache, of Leyland.  
This was dated 1332, in the Lay Subsidy Rolls of Lancashire, during the reign of King Edward 111rd of England,
1327 - 1377."

1480: Geoffrey Travers (henchman of the Earls of Derby) held Blythe Hall, ORMSKIRK since 1189.
His line ended with Roger, whose daughter, Margaret,  married John Blackledge in the year 1480.
(Probably from the family once living at Leyland Old Hall.)

1535: Nicholas Blackleech of Leyland, was "doctor of phisick to King Henry VIII" in 1535.

The spelling Blackledge is particularly well recorded in the church registers of Lancashire from the late 16th century.
Here are two examples:

1565 Alicia Blackledge on May 15th 1565, married a Willmus Wyton at the village of Standish

1579 On November 27th 1579, Richard, the son of Ewan Blackledge, was christened at Ormskirk.

Ref: https://www.surnamedb.com/Surname/Blackledge