(it’s really somewhat more complicated than that but let’s keep it simple for the purpose)
Don’t try to think it in the range 0-255, think as everything is normalized in [0…1] range.
So, BlendFunc(ONE, ONE) does
colorOut = actualColor * (1 1 1 1) + colorIn * (1 1 1 1)
colorOut = min(colorOut, 1);
Result is so called ‘additive blending’, good for sparks and energy like stuff.
BlendFunc(SRC_ALPHA, ONE_MINUS_SRC_ALPHA)
Is standard transparency (well, I realize I might have swapped the two tokens). So, the result is a mixture of the color already in framebuffer and the color which the video card is trying to write in this render pass.
Since I also hardly remember the behaviour of this func, I usually like to read something from this , it has some interesting examples which can be useful for beginners.