Your post title focuses on slavery but your comment concentrates more on your paternal grandfather's case specifically. There is a potential answer that does not involve slavery. In the latter half of the 19th century and early 20th century there was an important immigration of peoples from the British Antilles as 'braceros' to Cuba. This is why numerous black Cubans have British names and some even play cricket!

What attracted people to Cuba, both from Spain and the British Antilles, was a sugar boom that only petered out definitively by the end of the 1920s. This sugar boom coincides, more or less, with Venezuela's first oil boom, which left oil as their principal export.

Given the economic stagnation of several of the British Antilles and the potential social mobility that a boom provided, it would make sense that not all immigrants went to just Cuba but would also go to Venezuela, which is also along the Caribbean basin. The term used for them would be 'braceros', in Cuba, but I'm not sure about Venezuela.

A book about Caribbean immigrants from the British Antilles seems to have come out relatively recently, under the name Radical Moves (2013), by Lara Putnam. Reviews state explicitly that 'Venezuelan oilfields' are a topic covered by the book. You could try reading that book on your own and/or perhaps trying to write her directly to see what she thinks, since it is a very specialized field and she seems to be at the top of her game.

Hope it helps!