Some wiz likely got past the finish through the cracks between the individual boards and to the wood itself. Pet urine pulls out the tannins in the wood. That’s what the black crud is. There’s a couple ways to try and get that staining out but if it’s underneath the finish you will likely have to sand the finish off in the affected area to try and remedy it if a topical attempt doesn’t do the trick.
Do a google search for how to remove urine stains from hardwood and give those a try first, you just might get lucky. Looks like that floor is ready for a little touch up anyway. Hopefully it’s not a rental and they try and ding you for it.