Another theory supposedly contradicting the equally unproven claim that Neanderthals were blood type O negative is the one, that Cro-Magnons were rh negative.
In short terms:
There is zero evidence of that anywhere.
All you will be able to read online are fabrications.
Who exactly the Cro-Magnons were in itself is up for debate and their blood types were definitely not known.
As for the Neanderthals:
2 eastern specimens examined were rh +/+.
The Spanish examinations were revealed as O, no rh indicated.
There are certain similarities between Neanderthals and rh negative populations today. That doesn’t mean they were rh-.
A lot is yet to be examined.