blackheart vs. onslaught. who wins.?

Onslaught possessed the combined abilities of Professor X, Magneto, Franklin Richards and X-Man, including telepathy, telekinesis, astral projection, illusion creation, mental bolt projection, sensing mutant presences, manipulating magnetic fields, and affecting reality itself. Onslaught could also enhance its physical size and strength.

Franklin Richards by himself is on par with Galactus OR Elders of the Universe

Blackheart He's totally invulnerable to anything except the most powerful forces in the universe. That would be Onslaught

Onslaught period, he's one of 2 of the most devastating characters in the entire Marvel world. Thanos is the other.

Blackheart

Blackheart.