The Bluestacks Android emulator is built using .Net?

Android is Java-based, but that does not mean an Android emulator has to be written in Java -- just functionally emulate Android devices.

While BlueStack's site does not list any system requirements for its emulator that I could find, at least a couple sites -- e.g. Bazooka Apps and Android Emulator for PC -- mention .NET as a requirement.

Ultimately the proof of what's going on is right in the process list and Reflector results you found.