This khronos OpenVG extension lists a bunch of alpha blending equations that are even more complex than you'll get from OpenGL blend operations:
khr_advanced_blending
The w3c compositing and blending specification goes over much of the same stuff, but also has pretty pictures:
w3c compositing and blending
All that said, most of the time you'll get by with "weighted average" (like in episode 38), "saturated add" (for explosions, fire and other and glowing stuff; saturated means that values cap at 1.0) and "multiply" (for shadows).
khr_advanced_blending
The w3c compositing and blending specification goes over much of the same stuff, but also has pretty pictures:
w3c compositing and blending
All that said, most of the time you'll get by with "weighted average" (like in episode 38), "saturated add" (for explosions, fire and other and glowing stuff; saturated means that values cap at 1.0) and "multiply" (for shadows).