blendfunc grey + grey = black?

Hello, excuse my bad english in advance…

I’ve a problem:
I wanna make some shadows on a plane, and there are some shadow-objects which overlap. I wanne make them kind of grey and when they overlap they should get darker. I am trying to use blending for solving this. I know there is the math-function, behind it, and i’ve seen this constants which i can give at blendfunc. (i tried them randomly). Before i get out my calculator, does anybody know how to get the blendfunc to make quite the opposite. I wanna have the objects become darker when they overlap, but they turn brighter.

regards && tia

This is only a guess, but could you pass a negative value into one or both parameters of glBlendFunc?

I tried this with a lot of modfications.

One thing i would need is to reverse the colours. What is brighter needs to be darker etc.