you are not blitting attachments, you are blitting the contents of your FBO
in this case you are blitting the color content of your FBO to another FBO
at the same time you are also copying the depth data to another (same destination) FBO
unfortunately i don't know how to copy contents of textures to FBOs or other textures
because that's what attachments are: textures, or (storage) "formats"
but i'm sure someone will confirm
I hate answering my own questions but I'll put this here in case anyone needs it. I came up with an ugly solution that involves doing two blits changing glDrawBuffer in between.