I imagine the blending would occur during moments of slaughter. One tribe attacks another - kills all the adults/men and takes the children/young women - and vice versa.
Over many many generations, over many tribes across europe, this would ultimately lead to blending.
If you are killing off all the Neanderthal men you are not "absorbing" them.
If Neanderthals also could do the opposite, I guess it'd be called 'absorbing', since it's more fair.
But this also would need humans accepting 50/50 children in the tribe.
When you spell it out, it doesn't make sense. This is why I'd want to know how you do this.
Adding Neanderthal women to your slaves and killing the men, and keeping them there multi-generational until they become blended makes more sense to me for instance. But that's wiping them out, not absorbing them.
Over many many generations, over many tribes across europe, this would ultimately lead to blending.