For anyone who comes across this in the future, I reached out to blockchain.com (which was blockchain.info) and they tell me that it is indeed the number of unique addresses used during that day for either sending or receiving. They also told me they'd update it.. but haven't yet and it's a couple months later.. someone else want to poke them?
don't get your hopes up. blockchain team is one of the laziest ones in the world when it comes to changing or updating their platforms even when it comes to correcting the bugs in their platform including their wallet! there currently are a ton of issues that have existed for nearly as long as the site existed and they are well aware of them and yet don't move a finger correcting them.