My boat covers are always light grey to keep the thermal heating down.

I dont think a dark blue cover will ever get hot enough to damage your hulls, but you could always put a thermometer under them and measure it and report back.

Uncovered under the hot sun is bad too, so unless it gets above 200 under the cover, I wouldnt worry about it.

Leaving boat parts in the car on a hot day is way hotter than something left outside. I worry more about about stuff in my car than stuff sitting under a cover in the sun.