is Darkheart Sliver black/green creature considered a nonblack creature?

[Swamp]

Is a black/green creature (such as Darkheart Sliver counted as a non black creature (and can for example be used as a target for Doom Blade?

[Arix]

No. "Nonblack" means exactly that - "not black". When you ask Darkheart Sliver what colour it is, it replies "black and green" (and then you feel weird that a card is talking to you, but anyway). Just because it's a colour other than black as well, that doesn't mean it's not black.

[Swamp]

How about Arsenal Thresher if i pay 2WU?

[Arix]

Nope, that's still black. No matter what you pay for a hybrid card, it is all colours in its cost.

[guyarney]

Good questions though. I think alot of players ask themselves the same question - I know I did.

Simple rule of thumb: A card is all of the colors of mana symbols in its casting cost, period. If there are no colored mana symbols in a card's casting cost, it is colorless.

Though in MAGIC, there is always an exception, such as Transguild Courier.

[Arix]

Another good rule of thumb is that every card is either black or nonblack, and no card is both. If it has black in the cost, then it's black, and therefore it isn't nonblack. That can also apply to other colours, as well as other things like card type.

[guyarney]

No card is both yet...

But you make a good point. One could easily think that a card such as Beseech the Queen is both black and colorless.

[Swamp]

If there was a darkest hour in play would the b/g sliver still considered green as well?

[Arix]

Nope. Darkest Hour strips away all other colours and makes creatures black and only black.