Ah, yes, you're right. I keep forgetting this crap!
White is the combination of light on all wavelengths without absorption, and
black is the absence or complete absorption of all light.
So rather than Umbrakinesis, it could be used for invisibility (just producing dark matter), or telekinesis (manipulating dark matter).
Though, my hypothetical scotons are still viable, by that logic.
“…Judge not what a man has done, but judge what he could have done if he was a different bloke altogether. For art thou a leper? And a leper can changeth his spots…” --Rudy Wade, Misfits (Series 4, Episode 8)