Well, if you have an alpha channel in the framebuffer (the engine requests only R, G & B channels explicitly, so that is not guaranteed !), then it works just as specified by the blend func, i.e. it works just like the color channels only on the alpha channel you have SRC_COLOR == SRC_ALPHA and DST_COLOR == DST_ALPHA. If you got no alpha channel it will always be 1.0.
You can see the detailed specification here
, but you better have a MathML enabled browser.
If you have no texture bound, it is practically the same as if you had bound a texture that is constant white (RGBA=1,1,1,1).