Thank y'all for helpful tips and advice regarding my black squirrel problem.
Since my wife and I took actual measures to deter or catch them,
we have not seen any of those black ground squirrels.
The trouble is that we can not figure out what happened.

We deployed too many countermeasures simultaneously
to determine which, if any, had a real impact.
My wife tried fox urine and underground ultra-sonic noise maker.
So we don't know if any of them really worked, or if these squirrels
simply picked up and left for some other reason.

I set up a Havahart trap with cereal and peanut butter hanging in the air.
I also sprinkled peanuts on the ground leading up to the trap,
and put some more down inside the trap all the way to the back of the cage.
Much to my surprise, everything was gone, including peanuts inside the trap.
Even those in the back (and inside) of the trap were gone.
I tested the trigger mechanism several times with my hands before I set the
trap.
It wasn't the lightest or most sensitive, but I thought an average squirrel
would
trip it. Is it possible that these squirrels are not as heavy as I thought?
Or did some other lighter animal (even bird) eat the bait?
Is it possible that these squirrels are smarter than the Havahart engineer
and defeated the trap?

Ever since we actually took measures to deter or trap them,
we have not seen any of them (they are a family of 4, I think.)
Is it possible that they possess psychic power, and read our mind that
we are out to get them (and are messing with our mind)?

I hope I don't turn into the Bill Murray character in the Caddy Shack.