Never happening with my Necrons. My Necrons would more likely ally with the Tyranids to beat the Blood Angels. Necrons aren't made out of biomass, so Tyranids generally ignore them. It's only because the Necrons see the Tyranids as vermin that they actually fight each other at all. In actual fact the Tyranids are the Necrons' key to reclaiming their lost empire, if the Necrons actually allow the Tyranids to do their thing, as the Necrons can just wait for the Tyranids to eat all of the lesser races to leave barren worlds behind for the Necrons to annex for their Empire and build Tomb Complexes on. The Tyranids will ignore the Necrons and once they've eaten every last remnant of all the other races, they'll move onto the next unfortunate galaxy to be consumed, so the Necrons will have the whole galaxy as their empire. Just as planned.