It seems Blitz does not z-order alpha textures, which is IMHO really, really bad.
It's not really Blitz's fault, but rather HW and DX related "problem". Most 3D-engines (even games) have the same problem and because of the nature of alpha, there is no automatic way to z-order it. Only way to do this is to sort every polygon "manually" in every frame, and that is VERY cpu-time consuming. I doubt that DarkBasic for example is any better in this matter.