When measuring gold what are the black flecks in the gold?

Maybe gnats.

Maybe it is black gold.
https://www.diamondere.com/p/mens-rings-norval-1136-1133?metal=10k-black-gold&stones=emerald&utm_source=Google&utm_medium=PPC&utm_campaign=793460301&gad=1&gclid=CjwKCAjwkLCkBhA9EiwAka9QRnwRg3Uaraa7PPwh2rtKHP2oJmD_qpxFkBL7zI5eedVmCzuFznVV5RoCEA8QAvD_BwE

Why someone would prefer black jewelry over other bright and shiny colors of gold is another good question.

It’s sediment aka sand or dirt. An experienced panner knows how to wash it out properly. The gold will fall to the bottom of the pan at the same time as the sand/dirt washes away if you know how to pan properly.

Fill your classifier with loose gravel and dirt using a shovel. Once it is almost full, place the pan and classifier together under water, and move them in a circular motion to allow light sediment to float away and the heavier material to sink to the bottom of the pan.

Read here for more info

It could be lead or silver, which are both heavy and difficult to separate from gold flake when using a sluice box or gold pan. That, and lead and silver are sometimes found with gold. I have encountered this and could also be some heavy mineral compound, such as what you might find in granite.
Yes it does affect the price. The true value of the gold can only be known after the gold is smelted with borax, which will mix with the black sands but not with the gold. The gold separates out of the borax like oil separates from water. (Lead and silver will also not mix with borax.)