Why was he able to? Isn't the black box supposed to be a black box?

The black box doesn’t transmit its location. If they find it, it would/will tell you some of what happened, but if you sink it in the Mariana Trench and they can’t find it, you’ve “won”.

The black box can also transmit its location, albeit only after the plane hits the ocean: https://en.wikipedia.org/wiki/Underwater_locator_beacon

The black box has nothing to do with the transponders and tracking equipment.

The black box can’t be disabled AFAIK.

The argument in favor of disabling tracking is that the equipment itself could potentially malfunction in a way that disabling it would be safer (if it were interfering with voice communications or transmitting an emergency code inappropriately).

I fly small planes and it is not unheard of for ATC to ask pilots to cycle the power on a transponder that isn’t responding as expected. I assume that airline grade equipment is more reliable, but the same principle applies

AFAIK, the FDR and CVR can both be disabled by crew to some degree. I think they can be disconnected from the aircraft's power system (by fuse/breaker), but they also have some amount of battery backup.

Part of Federal Express Flight 705's whole debacle was that the hijacker tried to pull the breaker on the CVR.

And the investigation into SilkAir 185 was hampered because the recorders had their power pulled as a apart of load shedding while trying to resolve issues onboard the aircraft.

There are two components, one is the Cockpit Voice Recorder which can be disabled because it's powered from the main system and records on a 2 hr loop that may need to be preserved if there's some incident that isn't a crash. The other is the Flight Data Recorder which isn't on a circuit breaker and records on a similar loop but is more of a crash system so they just run until there's no power on the plane for some reason.