They’re called the Slave Narratives and there are hundreds of individual interviews done with former slaves as part of the WPA in the 1930’s. I’ve read a bunch of them, nowhere near all, and they’re fascinating. And yes, in many cases the former slaves speak fondly of their former owners and recount many small acts of kindness that were shown to them. Others talk of more strict owners, and a very small minority recall owners who were downright cruel.
It seems that most of the interviewers were white so some of the former slaves may have been guarded in what they said. In some cases they may have feared that if they were too outspoken they would lose government benefits.
One case was found where a woman was accidentally interviewed twice--once by a white interviewer and once by a black interviewer. When speaking to the white person she made it sound like she had been treated well. When talking to a young black man she laid it on thick how bad it had been for the slaves.
As someone on these threads noted years ago, the one thing you won’t find in the narratives is anyone wishing they were still a slave.