Under DirectX 9, it was still necessary to query the device for the capability bit “post pixel shader blending” on a per-texture-basis.
This functionality now doesn’t exist any more, but DirectX 11 has the whole new Output-Merger_State OM, which basically does what PPSB says on the tin.
I can’t find anywhere that it says that DX10 and DX11 guarantee that they offer this capability, so can I always rely on it?
AFAIK yes, it’s always available.