What if the Spanish authorities had allowed/encouraged slavery and the importation of slaves to the extent that the British and Portuguese empires did? Which parts of Spanish America would be most ripe for large scale plantations? Which parts would be majority black today?
None would be majority.
The problem with importing slaves is the tasks that labour is needed for in Spanish America, mining and tropical plantations, are extremely unhealthy and anti-family and would generate a constant population loss amongst the slaves.
The only part of the Americas that have large black populations are those where conditions were survivable enough to see slave population growth (American subtropical and temperate plantation and parts of Brazil), or where there was no other population base (the Caribbean). Neither of those conditions would occur in any part of Spanish South America; slaves would be worked to death and outgrown by the free locals.