some stuff that works with this thing:
The GBcartBreakout is a board that fits with one end inside a DMG/GB, GBC, GBA or NDS (slot 2) and accepts those same cartrige formats in a slot on the other end. Between that it has a set of DIP switches to allow passing through those cartridges' signals between the board's "contact fingers" and the slot, and a 100mil male header with each signal of each side broken out nicely.
Also there's a reset button across the fingers' reset and ground line and an additional pullup (10K) resistor protecting the slot's reset line (because Nintendo usually just shorts that to VCC in their cartridges and you don't want your GameBoy or batteries to blow up when you press the button).
sadly, it doesn't turn blue. but you can use it to:
essentially it allows you to interface with either one of or both the game and the system in a way that saves you from always building custom PCBs to fit your stuff when you're still researching/tinkering around (e.g. on a breadboard).
and yes, you can fix the aforementioned thing it doesn't do by just connecting your own blue light to the handy header.
Build your own from those files or contact me.
see this page for kit soldering instructions and a list of required parts.
this is a shot of the first one ever built.
(shown without cartridge slot at the top, but you can see where it'd be)