This isn't really a white card, and the flavor text is very, very .

White gets to tap down creatures (though not as well as blue) black certainly gets to put -1/-1 on things.

It's probably fine, the times that a -1/-1 won't actually kill the thing, it's either not gonna matter (because you intend to convert it to something else) or you're gonna be annoyed that it's tapped 'cause you wanna get into combat with it to finish it off.

Actually, this is just as bad color-wise. Hybrid cards need to make sense in either color. White can tap, black can put -1/-1 counters on things, but usually, neither can do both. This was, of course, violated in the Scars of Mirrodin block, since Phyrexians of all colors bled a bit toward black. See Choking Fumes.

This is mono-black. Extort in Orzhov was a color bleed.

Blocks cannot mix +1/+1 and -1/-1 counters. I suppose this could theoretically be printed in a product like Commander or Planechase, but it would be , not .

This effect does not and should not appear in black. In White, in a set that stretches all colors toward black, like New Phyrexia, this would be okay. It's rather similar to Vapor Snag, in that way.

This seems fine in monowhite common to me (in a colourbleed set like this one). Cf Erase and Divine Offering. The "lose 1 life" rider is very dubious in a normal set but par for the course in a bleed-towards-black set like New Phyrexia, as Link said.