Blend Bug? help!!

[rixarn]

Hi everyone.. im really tired of trying to figure out this by myself so I humbly ask for some expert's advice

OK its as simple as this:

Blend tool doesnt work at all. Well, it does but i cant see the blending. All I see is black where the blending should be.

Is this a videocard related problem? or it is a bug in the program? thanks in advance!