Like a clot, mixed black in with blue (6)

I believe the answer is:

cobalt

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'blue' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for cobalt that I've seen before include "Silvery-white metal used in alloys" , "Hard ferromagnetic element, Co" , "Deep shade of 16 dn" , "Metal giving its name to type of deep blue" , "Deep blue colour" .)

I've seen this clue in the Evening Standard.

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