You shouldn't be worried about what's blackhat, if you're doing any kind of link building such as guest posting then it's blackhat as well,
What you should worry about is the safety of the practice,
Multiple domain redirects used to work pretty well, but many domains have been doomed and it's simply not safe anymore,
Don't bother with more than 2 of them, instead go for PBNs with these domains, they will still do the job