Multiple bitmaps/textures using the same alpha

I''m just starting my first terrain editor in DX8 (had one in DX7). I''ve come across the issue of one terrain converting into another, and for some types, it is acceptable to simply blend one into the other. This technique is going to be my default one for having a terrain next to another, so I''m wondering, how do I make it so that I can have a set of alpah channels for the blending that I can use for every texture. That way, I could use the same grass bitmap regardless of whether it is completely surrounded by grass, or if it has dirt all thew ay around it. All I''d have to do is select the appropriate alpha. My idea was to make a black bitmap (which is my invisible color in the color key), and have an alpha key on that, but I''m not familiar with using multiple textures. Is this the rout to go? Thanks for any input --Vic--