[RalphLambrecht]

Are clouds standing out against a darker sky the main concern in street photography?

I think 2F's suggestion of using a blue filter is good.
Shadows and shade have a bigger blue content, and using a yellow or orange filter deepens them even more.

But if, as was in the question, you like to darken a blue sky, there is no way round a yellow (or yellow-green) or orange filter.

He needs to expose and develop his film differently to control the contrast between shadows and sky and forget about the filter idea altogether.