Let me just start with the 90 degrees outside issue. I don't think that even a single tiered cake will fare well outside in that kind of heat! It will melt/sweat/weep, and possibly the layers will start to slide off of each other.
Is there any way it can be kept in a/c or a cooler with cold packs until it's time for cutting?
If so, then as far as filling goes, you could mix raspberry puree into buttercream. That is tasty and easy, and stable.
I think that black...