Aokiji has a flashback of Blackbeard's invitation as a motivational driver to fight against Garp and says he has to protect Blackbeard's interests in his absence and goes on missions serving Blackbeard's goals, which includes even invading Yonko territories. And note how Blackbeard essentially gave him a job and a purpose, as he was a jobless wanderer, convincing him to join.
So it's not like Aokiji is "using" Blackbeard for his own hidden agenda. He may find his purpose in the course, but so far his interests allign with Blackbeard's interests in a sense of him being willing to serve.
And unlike Buggy, who was said to deserve the title of Emperor due to commanding the likes of Crocodile and Mihawk, Blackbeard wasn't really complimented on commanding Aokiji. He already was an established Yonko at that time, Aokiji joining Blackbeard at this point wasn't even a surprise in terms of power level, no one said that Blackbeard deserves to be a Yonko for commanding a former admiral and fleet admiral candidate, he already established himself before that.