Do you realize that in the 1950's only about 2% of the blacks in the South were registered to vote. In the South in the 50's you could lose your job, your house, are even be lynched for trying to vote. Most of the black voters both in the South and North were educated and in the middle or upper class. They were certainly not representative of blacks at that time.Who claimed that? The point is the strategy, not the outcome.
And you left out Texas.
Texas isn't part of what's traditionally called the south. Since most of the southern states have democrat majorities, I'd say the "strategy" is someone's fictionalized creation, rather than fact. Why make an issue of federal elections when the Republicans can't even get majorities in the state legislatures?
Are Republicans now getting 10% of the black vote, since the strategy began? Yes.
Did you know Eisenhower got 39% of the black vote in 1956? Probably not.
Did you know Goldwater voted against the Civil Rights Act of 1964, and other than his home state of AZ, only won some states in the Old South, aka, the formerly Solid South, as in solidly Democrat?
The Goldwater and Johnson's run for the presidency was in 1964 which was also the year the Civil Rights act passed which broke the Solid South. It is not surprising that Goldwater would carry a few southern states. His stand on Civil Rights was confusing at best. True, he voted against the 1964 bill, but supported the 1957 and 1960 bill. He was a member of the NAACP but he was also a very strong states right supporter. In other words the Southern votes for Goldwater were a vote for states rights.