In case you wonder why black sand is considered a good indicator for gold it's very simple. It only has to do with the fact that black sand is heavy and so is gold.
Black sand or any heavies including gold, will drop out of suspension in lower pressure areas where the water flow slows down. Usually around inside bends of a water course, behind large boulders, drop offs, where the water way widens, etc. But since gold is much heavier than black sand it usually is found more toward the bottom closer to bedrock, as well as in the cracks and crevices of the bedrock.
When gold gets washed into a waterway it has already started to sink through any gravels or mud or sand on it's way to the bedrock but because of turbulence it keeps getting tumbled into the mix and will continue downstream that way until it becomes trapped, or if the water has slowed enough to end it's momentum. The exception is that the really fine and flour gold along with black sand that has recently been deposited by a flood type event will not have settled as deeply as larger flakes, pickers, or nuggets of gold.
I've found that when a good rich deposit of black sand is seen close to the surface such as commonly seen on sand bars, it's best to dig on the creek side of the deposit due to the way the heavier gold travels vs the lighter black sand.
In your case, that black sand that you saw in the bank seems to have been exposed through erosion of the bank. I would look for other indicators such as gravels that are also imbedded into the bank, look for layers and sample between each layer to find a pay streak as all layers may not contain any gold, only a specific layer may contain any appreciable amount of gold.
But that is what prospecting is about, searching for a pay streak, once found your job changes from prospecting to mining
The other thing is that all black streaks seen from a distance along streams and rivers may not be due to a presence of black sand but could just as easily be due to organic decomposition staining the surrounding material causing it to appear to be a black sand deposit, you will need to get up close and personal to be sure.
Go for the gold,
GG~