Green’s Dictionary of Slang

razor n.2

also razor-blade
[rhy. sl.; razor blade = spade n. (1)]

a black person.

[UK]G.F. Newman Sir, You Bastard 225: He called you a razor-blade.
[Ire]J. Morrow Confessions of Proinsias O’Toole 26: On many a rough Saturday night when razors cleft the marijuana fumes.