I'm pretty sure Direct3D changes the FPU to single precision mode.
In order to use doubles in a Blitz3D program the DLL would have to allow mode changes. You would switch to double precision mode, do your calculations and then switch back to single precision.
In order to use doubles in a Blitz3D program the DLL would have to allow mode changes. You would switch to double precision mode, do your calculations and then switch back to single precision.