Inside the Darkforce dimension would favor Blackheart greatly.Goku loses.Outside of it, Blackheart isn't a being to loss via fist fight.He took blows from Mephisto without feeling pain whatsoever.The thing is, this happened inside Mephisto's own realm.The only thing Goku has that could hurt Blackheart, is that spirit bomb.Though I wouldn't count on that to be a way for Goku to win.
If anyone want to know what Blackheart capable of.
Here it is
Blackheart is the son of Mephisto possessing vast supernatural powers and immense magical and mystical powers over reality and dimension including energy projections and manifestations, magical shields, intangibility, invisibility, control over the elements and matter as well as darkforce energy, summoning legions of demonic hordes and creatures, healing and repairing matter and living tissue, manipulating minds by telepathic means, and even telekinesis, casting illusions, teleportation, dimensional travel, and cast shape-shifting.He is believed to be omnipotent and mystically linked to the Darkforce dimension. He can steal souls without one's approval unlike his father. Blackheart is able to lift over 100 tons.
There also more details of Blackheart Powers and Abilities in this site. Here the link