Disable color rendering on WebGLRenderTarget - possible?

Hey there, I wonder if there is a way to disable the color buffer on a WebGLRenderTarget?

I would like to render scene A (depth only). Then I would like to render scene B but with depth information from scene A in order to be able to hide certain parts of scene B.
Later on, I will blur scene B to achieve a bloom effect.

Then, I would like to render scene A normally and put the blurred scene B over it.

Any help is greatly appreciated.