Good question, on the surface it does certainly seem implausible that slaveowners selectively bred their slaves to the point where superior physical traits were established. That said, there is a little more to the question than meets the eye.
Firstly, bear in mind how these slaves ame to be in the americas. They were sold to traders in (mostly) african states, then floated across the atlantic in very spartan conditions. This is relevant for two main reasons:
the people being traded as slaves were people that had been selected by traders
the slaves that actually made it to the americas were the ones that survived the trip
So it seems very likely from this that only the most resilient/strong/healthy slaves were making it to the new world.
Once in the americas it is likely that slaves were selectively bred to some degree - the process would have been more than familiar to the plantation owners, and there is some evidence to suggest this went on. It's also worth remembering that traits can be selectively bred into populations surprisingly quickly. even in a very small number of generations (see the fox breeding programs in russia and other examples).
With all this in mind it is effectively certain that a degree of change or selective breeding in slave populations went on, however, the degree to which this effects modern populations is debatable for two main reasons:
genetic diversity among humans is very, very low to begin with compared with almost all other species, therefore comparisons with other animals selectively bred are not really comparable, as there is less diversity to select from.
the modern black populations all have significant numbers of white and other ethnicity in their family trees and genetic makeup. In a very simplistic sense we can see this from the fact that blacks in the US do not look like the subsaharan africans the slaves would predominantly have been.
Final point I would make is that the "strongest" or healthiest populations from a biological standpoint are typically those with the greatest genetic diversity in them. With this in mind it is possible that populations in places like the US that have ancestors from several ethnic backgrounds are "healthier" biologically speaking than more isolated/inbred populations.
tldr: it's very complicated to answer, but there is a degree of truth in it, but the point to which it is practically relevant is very debatable.