Let me start by saying that I'm a big fan of Shanks.
He must have been pretty powerful before (otherwise WB wouldn't have recognised him) and even more so after he became captain (he must have gain tons of battle experience if he were to have gained the title of one of the Yonkou).
And it's probable that he got his scar before he became captain.
And he might even have gotten more powerful after losing his arm (think Yang Guo of Return of the Condor Heroes by Louis Cha. For those who don't know, Yang Guo, after losing his right arm, trained his left arm against torrential currents for about 10 years and gained tremendous strength that made him one of the strongest in China at the age of 36). Of course, this is merely speculation.
My main point is, despite this, I think Blackbeard is EVEN MORE powerful. First, he gave Shanks a scar that distinguished him against other pirates in his career. Second, he gained a crew of 4 more people who could handily defeat an entire country on its own. Third, Shanks, who probably heard of "Fire Fist" Ace, probably knew Ace's pure-pwnage Logia powers and yet knew that he couldn't be pitted against Blackbeard, probably only gauging Blackbeard's powers with that moment in time when he fought him, almost 22 years ago. Fourth, Blackbeard learnt of the Darkness fruit and for some reason, he seemed to be the only one who knew about it (WB didn't seem to know). Fifth (pretty crappy, but anyway..) he survived Ace's Hiken, which could destroyed several ships in one go.
My conclusion is that Blackbeard should at least be of Shichibukai power, if not Yonkou-level, at the point just before he ate the Darkness Fruit.