Something non-obvious that I learned just the other day :-)
If you ever do a blit and wonder why it doesn't seem to work though no error occurs, check glDepthMask(), glColorMask() and glStencilMask(). They can prevent the blit from writing something into the destination buffer. Also be aware that it is possible to direct the writing of color data into several destination color buffers at once via glDrawBuffers().
Re: glBlitFramebuffer tip
Thanks. Something to keep in mind.