A clot to mix black in with the blue (6)
I believe the answer is:
cobalt
'the blue' is the definition.
(cobalt blue is a kind of blue)
'a clot to mix black in' is the wordplay.
'to mix' indicates anagramming the letters.
'black' becomes 'b' (as in pencil classifications).
'in' means one lot of letters goes inside another.
'a'+'clot'='aclot'
'aclot' anagrammed gives 'coalt'.
'coalt' enclosing 'b' is 'COBALT'.
'with' is the link.
(Other definitions for cobalt that I've seen before include "Deep blue colour" , "Metallic element, Co" , "Silvery-white metal used in alloys" , "Bright blue pigment" , "Hard ferromagnetic element, Co" .)