Any sort of solid cover will help, even a cheap poly tarp. What you are trying to do is reduce the evaporation off the surface. That is the main driving element to pool heat loss.
My cover looks like the manufacturer bought up a bunch of black/blue poly tarps and cut/hem to circular shape.
Laid directly on the water -- darned thing floats if give the chance -- reduces heat loss to a minimum.
Before I started using it, heat loss closely followed ambient at night. With it, I'm losing only a few degrees.