What does this decoloration look like to you? Rot? Sunburn? Benign?
The cactus was cut some 10 days ago, and it was perfectly green at that time.
What actions, if any, should one take now?

It should be noted that the flesh is perfectly firm when pressed.

http://i.imgur.com/y6rGS.jpg

weird, looks like it got bleached by the sun. I think in most plants the suns radiation constantly destroys chlorophyll and the plant constantly replenishes it, so maybe with the stress of healing a wound it got a little behind in chlorophyll production? That's my crackpot theory. The plant looks healthy anyways so I wouldn't worry about it.

it does look like sun scald
it will be fine, just some cosmetic damage

EAT IT