The little rectangular intake manifold water jacket cross-over cover plate.
Is that the technical term?!!?
Sounds good enough to be the technical term.
I know what you mean about seeing coolant stains inside the supercharger inlet when you remove the throttle body. Sure, the coolant naturally flows as Will has stated, but the gasket is a weak point and unless you coat both sides with silicone ( just a light smear) you stand a good risk of leakage that could eventually get bad enough to do serious damage. Thank good GM design for making it leak on both sides, so you get some advance visual warning.
My guess is that the supercharger vacuum at the entrance is very high and tends to draw pressurized coolant thru the unsealed gasket in very small amounts that only progress worse.