How risky is to to do a 301 to an exired domain?

I know people generally buy expired domains to redirect them but my situation is different.

I picked up an expired domain with a solid backlink profile and decided to make a money site out of it in the exact same niche. Now I have an existing site in the exact same niche that i want to redirect to the expired domain so i can have just one big site...

how risky is that? is it super black hat? They are all in the exact same niche....

It's not risky at all, it is what 301 was intended for, to redirect traffic, and links to a new domain, it doesn't matter if that is an old domain, fresh domain or an expired domain.