Okay, look, you're gonna have to adjust your expectations a bit when it comes to 'free' and 'steamy'—especially for the mature ebony niche. Those two things don't tend to overlap legally very often.
My main move has always been Kindle Unlimited. It's not technically free, but for the monthly fee, it's a solid library. You'll find authors like Brenda Jackson, Zuri Day, and some of A.C. Arthur's older titles. They have plenty of seasoned heroines, often in their forties or fifties, dealing with second-chance romances or rekindled passions. The plots can get pretty heated.
For actually free stuff, Project Gutenberg has some classics from early 20th century Black authors, but obviously, the steam level is more implied than explicit. Your best bet for contemporary spice without paying is to follow authors you like on socials. They often run freebie promotions for the first book in a series to hook you, and I've snagged a few that way. Just be prepared to pay for the sequels.
Honestly, I've wasted too much time on sketchy download sites promising free ebooks only to get malware or badly formatted files. It's rarely worth the risk. Stick with the legit promos and subscriptions.