You have to specify an alpha channel for the alpha test to work.
To do this, when you load your texture as a RGB 24bpp bitmap, copy it to a buffer which has a RGBA 32 bpp depth. Copy RGB to RGB and for A put 0 if R=G=B=0 (black) and 1 otherwise. Then create a RGBA texture.
Then specify a value of say 0.5 for the alphatest’s trigger value and draw your textured geometry with alphatest enabled.
If you want to make the rest of the texture 70% transparent, do as above but when the pixel’s color is not black, put 0.3 for the A component instead of 1.
Then use blending instead of the alphatest (Probably the best blend function for that is glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)).
Alternatively, you can use a targa bitmap which already has the alpha channel and load it as the RGBA texture.