I wouldn't say that is a blackhat practice because it depends on why you would redirect this domain to your own site.
You can find types of response in this article. Extract:
1. Buying Expired Domains: Don’t Expect Credit
Have you picked up a domain that was once owned by someone, not
through buying it directly from them but because it had expired and
went back into the common pool of domains for purchase by anyone?
That’s an expired domain – and chances are, the backlinks aren’t going
to pass credit according to Matt’s statement.
2. Buying Domains & Redirecting Links: Probably No Credit
Did you see a tasty domain and think it would be nice to get it, in
order to obtain its links for a different site, such a redirecting
them? Sounds like there’s a good chance that Google is going to notice
the purchase date, take note of the redirection as well and decide
those “historic” links shouldn’t count. What about if you just paid
someone to keep the domain going under their name but closed down any
existing content and point to another location? Might work; then
again, Google might note the change, the oddity of one site to
completely point at another, and it might be that the links won’t
3. Buying Domain & Running Web Site As Usual: Credit Likely
Did you buy a web site from someone else and are maintaining the
business on that site as normal? Despite the fact that your domain
name registration will have changed, since the site is carrying on as
usual, there seems to be a good chance that link credit will continue
4. Getting Domains Through Acquisition: Credit Likely
Have a domain that changes hands, due to a company acquisition –
company A buys company B? You should be OK, thought it’s unclear how
Google tells the difference here from an ordinary transfer. And no,
Google wouldn’t share more on how they can tell the difference.