Look at the Castle demo in the Blitz 3D samples folder...it includes a little mario figure that can run around and such, pressing one of the CTRL keys makes him fire off a little fire ball thingy...this fireball is a sprite that is rendered useing the ADD blend mode (the source graphic for this sprite is in one of the folders)
Basicly how ADD works is instead of just coloring the rendered pixel with the particular texture color...it instead samples the background color and ADDs the texture color to it...if the texture color is full black (0,0,0) then the resulting color is the same (background color + 0 = background color...it just adds the texture color to the background color)...by playing around with not just the texture, but the entity alpha values too...you can use ADD for all sorts of effects from lens flare to smoke to magic spell casting effects
Basicly how ADD works is instead of just coloring the rendered pixel with the particular texture color...it instead samples the background color and ADDs the texture color to it...if the texture color is full black (0,0,0) then the resulting color is the same (background color + 0 = background color...it just adds the texture color to the background color)...by playing around with not just the texture, but the entity alpha values too...you can use ADD for all sorts of effects from lens flare to smoke to magic spell casting effects