I'll try for a short answer. Tell your grandson that our fabulous white sand here in South Walton came down from the Appalachian Mountains thousands of years ago. The quartz sand was affected by several different ice ages over 30 or so major time periods. Our white sand in South Walton and Northwest Florida is so special and different as during the Pleistocene period there was an epic event that caused the siliclastics (or quartz) to come to our beaches and the more carbonate mix to go to the Bahamas platform. Now, over those thousands of years there were other things going on like change in sea level, plant and animal impact, etc. Also, 10,000 years ago, the shoreline was 100 miles out from most of the Florida shore. Here, since we are also naturally higher than the rest of Florida, ours was probably out 40 or so miles at this point. And, our part of West Florida is probably from a whole different continent than the Peninsula part of Florida. (continental drift, etc.). As for the dark sand mixed in and exposed from time to time, BeachSi02 gave us the name of this once. Maybe he'll chime in. So with all this dynamic activity over thousands of years, it is not unusual to see other things mixed in, especially when uncovered by high wave activity. South Walton is particulary special in that we have many coastal dune lakes mixed in along our white quartz sand beaches.
You didn't say whether your grandson was 3 or 23. If he is 3, just tell him it got mixed in with our white sand a long time ago and the big waves have temporarily uncovered it. It is clean and natural.