I've mongered at Tokyo RC before. It was kinda an embarrassing hassle to pick up the girls from the lobby (elevators to guest rooms require key card), since the two elevator banks on the lobby floor, one set to guest floors and one set to ground level, are not well hidden and also see high traffic. One set is in view or nearly in view of the front desk, while the other set sees high traffic and doesn't have any seating or discrete place to stand. You could use the seats in the lobby, but those are directly in front of the concierge and front desk. That being said, nobody - including staff - is going to give you trouble any precisely because it's a 5 star hotel. On the other hand, you're going to already get more attention because you're black. Though, if you don't mind, then it won't matter.
To be honest, most girls working in Tokyo sex trade are working there for one reason and one reason only, and not because they are bored out of their mind between model shoots. THC is not a haven for 9's and 10's, not any more so than any other average agency. Diamond does not equate to looks, it is related to how often the girls work, how good she is at getting repeat customers. Further, as these girls are just a cross-section of Japanese girls looking to make money, some may look out of place in the RC lobby based on age, dress, the fact that they're unaccompanied. Diamond girls may blend in more, but only because they have more experience, not because they are quote un-quote higher-level.
BJ skill is not guaranteed, though perhaps diamond level girls at THC will be better due to experience. Lots of Japanese girls just lick the tip and put an inch in and go up and down, boring. You may not be able to make an appointment because they require a phone call once you enter the love hotel room, but this agency focuses on BJ:
https://www.cityheaven.net/tokyo/A1305/A130505/iramachion/. It might be possible doing it this way: book online through cityheaven, send the agency a message through cityheaven chat confirming your appointment and that you'll call when you check into your love hotel in Ikebukuro, then call out using the room phone and explain in English or simple Japanese.
I used to monger at 5 star hotels in Tokyo, including Park Hyatt, Andaz, Shangri-la, Conrad, Ritz-Carlton, Mandarin Oriental, and a bunch of 4 stars. But now I do not for the previous reasonings. My actual go-to is the Hyatt Regency, since guests don't need key card to go up the elevator, which is pretty rare in Tokyo.
If you're good-looking, you should go out to Shibuya bars and clubs and play your cards. Looks have nothing to do with how good your mongering chances are lol