Deep shade of blue (5)

I think it could be either:

Which length description are you looking for?:

cobalt

(Other definitions for cobalt that I've seen before include "Deep blue pigment" , "Ferromagnetic metal - shade of blue" , "given radiotherapy with this?" , "Deep shade of 16 dn" , "Silvery-white metal used in alloys" .)

I've seen this clue in The Times.

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