There are more instances in the background of inquisitors working with traitor marines than there are of blood anhels allying with necrons.
As mentioned previously another is stele.
And quite frankly given chaos gets guard allies to do chaos guard its ridiculous to say that a similar alliance to do radicals wou like d be against the spirit of the codex.
But of course I fully dont expect the book to be that awesome and people will find whatever justification for **** chaos that they want too