Is having two different websites, in which one has a link to the other, black hat seo?

For instance: Let’s say I own a tar roofing company, and I have a website for it which is xyzroofing.com . Then I’m fortunate enough to get the domain name tarroof.com, which has the keywords "tar roof". Those keywords get tons of hits each month, so I setup the tarroof.com website, which contains generic FAQ’s on tar roof systems. Then I post an ad with a link for xyzroofing.com on tarroof.com, that says, "tarroof.com site sponsored by xyzroofing.com." Would this type of scenario be black hat seo? In other words, tarroof.com is an information site of useful content, that also acts like an ad for my company.

Yes. It is considered "cheating" by the major Search Engines. If you only do it once, no one may notice.