You can do animation masking (what you are asking about; blending is a different thing) by having a couple invisible animated skeletons, and the visible character with the same skeleton but without animation. Play the two animations you wish to combine, and match the bone rotations every frame.
This is hardly a beginner task however so I'd put this off until after you have experience doing other, simpler stuff in Blitz3D.
This is hardly a beginner task however so I'd put this off until after you have experience doing other, simpler stuff in Blitz3D.