Black king you're initially finding dull (6)
I believe the answer is:
bleary
'dull' is the definition.
'black king you're initially' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'king' becomes 'lear' (King Lear).
'initially' suggests taking the first letters.
The initial letter of 'youre' is 'y'.
'b'+'lear'+'y'='BLEARY'
'finding' acts as a link.
(Other definitions for bleary that I've seen before include "with tired eyes" , "Blurred" , "Indistinct - hazy in outline" , "Tired and unfocused" , "Dulled with fatigue" .)