A few things:
Black oxide (BO) will 100% rust. The treatment itself is a controlled rust...that's what the "oxide" means. This should not ever be used for exterior hardware.
I doubt they used BO hardware. They likely used a black zinc flake or phosporus treated, which is common beyond words. However, they are available with different levels of topcoat to create different levels of rust inhibition. They likely grabbed a fastener with a low hour-to-rust value and ran with it. Yes, this reduces the fastener's cost.
Do not expect anything better from McMaster unless you look to pay up for a high hour-to-rust (>250) rated fastener.
Stainless is not truly stainless. It will rust, it just takes longer. Most fasteners are 304SS - which is a pretty low grade. Lots of regular zinc coated fasteners have better rust resistance. Also, thread SS into raw aluminum and you can pretty much forget about ever getting it out as it they will nearly instantly gall together.