There were a couple of different color solutions for HyperCard, but the official one (Color Tools) was released with HyperCard 2.3. The window tint in System 7

There were a couple of different color solutions for HyperCard, but the official one (Color Tools) was released with HyperCard 2.3. The window tint in System 7 was a user preference. The purplish blue seen in the screenshot was the default color, but there were about a dozen other options. In any event, it seems a stretch to assume that Apple's choice of this color influenced Mosaic in choosing a different blue color for a different purpose.