Taken from the TextureBlend manual:
"The texture blend mode determines how the texture will blend with the texture or polygon which is 'below' it. Texture 0 will blend with the polygons of the entity it is applied to. Texture 1 will blend with texture 0. Texture 2 will blend with texture 1. And so on. "
The commands you need to set the different texture layers are, if you're using brushes,
BrushTexture -
http://www.blitzbasic.co.nz/b3ddocs/command.php?name=BrushTexture&ref=3d_cat or if you're applying textures directly to entities,
EntityTexture -
http://www.blitzbasic.co.nz/b3ddocs/command.php?name=EntityTexture&ref=3d_cat .
The last option, 0-7, indicates the layer number. Take note of this bit:
A little note about multitexturing and slowdown. Graphics cards support a maximum amount of textures per object, which can be used with very little, if any, slowdown. For most cards this is two, but for a GeForce3 it is four. However, once you use more than this amount, Blitz will emulate the effect itself by duplicating objects and textures. Obviously, this may then cause slowdown.
+BlackD