Would it make sense to allow a default entry (t FACE) in `org-src-block-faces', to be applied to all src blocks without a more specific entry in that alist? There seems to be no other way to specify a face to be applied to all src blocks, and only src blocks.
I also wanted to point out two minor issues in org-src.el: - It refers to the `org-block' face without requiring `org-faces'. This is probably never a problem in org itself, but if an external package loads `org-src' in order to fontify some code blocks, that face might be missing. - The docstring of `org-src-block-faces' contains the word "respectability".
