For a definite answer, Bob should ask his tax advisor.
German freelance status ("Freiberufler") is a bit difficult to navigate, because legally speaking, this status can only be applied to contract work that requires a university degree, everything else is a regular business ("Gewerbe") that is taxed differently and requires you to join the chamber of commerce.
This has become a bit murky as there are freelance software developers without a degree (who should be careful about using the word "engineer") and the tax office seems to accept that, but I'm not entirely sure they are as lenient towards entertainers (which YT would fall under).
The way I understand the Blue Card FAQ, freelancing is not allowed for Blue Card holders, I'd consider that the bigger problem (but that's an immigration issue, not a tax issue).