What's going on with my Cattleya pseudobulb? (Discoloration)
 

I've had this plant for about a month. When I first got it, it was in bloom, but my dog knocked it off the table with his tail. Now I notice that the pseudobulb looks off. Is this some kind of rot or did the plant get injured when it fell? Should I unpot it, cut off the pseudobulb at the base, sterilize with hydrogen peroxide, and repot in fresh medium?

Yes, definitely remove that pseudobulb. This type of rot tends to spread so you will need to wipe the cutting point with Isopropyl alcohol, then get a new cotton ball and wipe down the rest of the orchid (except the roots...the alcohol would kill them). Keep the orchid dry and watch closely. You may see another pseudobulb show signs of infection.

If you do stop this infection and take the orchid from its pot, I would stake it and the roots above the medium or not add medium until a new growth appears. So often, the roots become damaged while changing the medium and die from infection (they rot). Without new roots to support the orchid, it then suffers. By not surrounding the roots with medium, they have a chance to heal and continue to support the orchid.

Ok, so I just performed orchid surgery and repot. I took pictures of the process and put it below, including the cut area where the pseudobulb was.

It was pretty moist and rotten in there, I must have been overwatering it since I got it a month ago. Good to know, because I have a couple other cats I've been treating the same way. I used a clear pot that was slightly smaller, because I like having the option of seeing the roots. The media is bark chip, charcoal, and perlite, and I picked out the bits of sphagnum moss. I also potted it higher up than it was before.

It seems anchored well enough but is a bit top-heavy, so I put the pot in a small clay pot to help hold it upright. The rest of the plant (excluding roots) was wiped down with rubbing alcohol as you described.

How'd I do? Anything else going forward? Should I separate this orchid from the collection?

Rot like that usually enters the roots and ascends from below. Some would have kept cutting off growths and examining the rhizome until no rot could be seen.

In the Beginner forum there is a sticky thread about using wooden skewers to check wetness. Cattleyas should get dry between waterings.

Sometimes rot gets into a pseudobulb and it isn't clear whether root issues are cause or effect... Looking at where you removed the bad pseudoblub, seems like there is a little nub of rhizome left, and where it was cut was brown. If you can cut that nub down a little more without getting into the (apparently) healthy pseudobulb, I'd suggest taking the surgery a little farther. (single-edge razor blade is great for the purpose... makes a very clean cut) If you can get to tissue that isn't discolored, your odds of stopping the infection in its tracks improve greatly. Unfortunately, if it gets into the rhizome the plant will perish no matter what you try to do, so it's better to err on the aggressive side.