Alpha channels can be created in DXTBmp in the ways you have already tried. "black" creates an alpha channel where all black areas in the main image are placed in the alpha as 100% transparent with all else opaque. "Template" creates an alpha channel using a greyscale version of the main image (useful as a map for further editing)

Either method gives you an alpha which you ca the edit (send to editor) as muh as you like

However.....

In order to retain the alpha channel when you save the texture you MUST use one of the formats that supports alphas (DXT3, 32 bit, 16 bit 444). If you choose a format that only supports 1 bit alpha (DXT1, 16 bit 555) then you will just get a 2 colour on/off alpha. If you use formats that have no alpha (other 16bit formats, 24 bit etc) then the alpha will be lost