Yeah, I ran a few searches... couldn't find my answer.

I just busted my power tube in my Blackheart, but I was thinking about getting new tubes beforehand anyways. I only need one power tube, but when ordering from tubedepot.com, it says I'd be ordering them in a matched pair. Will I be able to just use one from the pair and then have another for replacement later? Or are matched pairs only usable when together?

Forgive the tube technology ignorance.

You should replace both. it is generally not advised to have mix matched tubes regardless of what the pros have. replace both at the same time and have them biased.
Gibson 58 VOS, Gibson Rich Robinson ES-335, Fender Strat, Fender RoadWorn 50's Tele, Gibson LP Jr Special

Marshall JTM45, Fender BJR NOS
Matched just means they run better when you have AB power. It will certainly be compatible and run fine but if you replace both at once then they are considered "calibrated" if you will, to run better.
1990 Gibson LP Studio
2007 American Deluxe Stratocaster
2006 Taylor 510-ce
Fender Geddy Lee Jazz Bass
Mesa/Boogie DC-3
Yeah, Blackhearts only have one power tube. Thanks for your time guys! Thanks for the answer, acdc.