Madonna hit the very top echelon/icon status a long time ago because she made a shitload of money for a long time and kept her name in the press. She's beyond trend cycles now and above the industry. Anything she does for the rest of her life will be of interest - she's a pop music Marilyn Monroe.

Janet has had a nice respectable career but even in her prime she wasn't as big as Madonna, and she's now well past it. The sexual double standard you're alluding to is actually very easy to understand: Madonna's thing is now two decades old and she's reached elder statesman status. All is forgiven. Janet's thing is much more recent AND it happened on live TV, watched by zillions of people, costing lots of people a lot of money in fines; naturally there was backlash. If Janet had just quietly put out a Girls Gone Wild video of herself or something, there's a minor stir and a month later, no one cares. That's the difference. (The REAL double standard there is how nobody came down on Timberlake - it took two to expose that titty. But he's white AND male AND it wasn't his stuff on display...)

If you want to see black artists who were at Madonna's level, two of them came out of the eighties alongside her: Michael and Prince. Prince just did the Super Bowl to raves despite being a sexual freak for his entire career (and playing live in pants with the butt cut out!) and not having had a real hit album in about a hundred years. Michael could very likely still have gotten the Super Bowl up until his death, despite the allegations following him around. Make the industry enough money and they won't care what color or gender you are, not really.

It's been a while since a black woman had anything like Madonna status though. Mariah is probably closest. But to be fair, no white women have that status either. We can talk about Gaga if she's still doing it in two decades, but for now she's just aspiring. Madonna's the only female star in her weight class since she came on the scene - that really says it all.