Tas
I have not used the VSF Black but have quite extensive use of VSF caps in electronics, inductors, caps in crossovers, CAST in electronics, CAST in crossovers and all Deulund is great. I am sure you will love the black. Sometimes I use like you VSF in electronics due to space.
I have heard the Black is in the middle from VSF to CAST and I think originally made to be in th middle for cost and sound reasons from VSF to CAST.
I have found you can intermix new copper foil in oil of different brands and even vintage. Now metalized poly caps... (no in my mind)
The difference in foil caps is the quality of the metal aluminum, copper, silver and the the resonance and it's control. You have low resonance caps Jensen Paper foil in oil VSF and lowest of all CAST and you have high resonance caps new ones like Ampohm and vintage fall into this category.
Resonance in some spots is worse than others. Tweeter caps need low resonance. Phono stage and pre-amp need low resonance.
Mid range? Resonance does add fullness to the freq that resonance occurs. I did not find a huge difference in VSF to vintage in the midrange. (like to do that test again sometime)
Woofer oddly enough you want a low resonance part there. Wish I had CAST but VSF was much better than vintage.