1. The legal definition of what's "commercial" under that law and what's not isn't quite as straightforward as a layman might think. (For one, that law doesn't literally say "commercial" ("gewerbsmäßig"), but rather "geschäftsmäßig" ("business-like"), which doesn't require an intent to make money, but may include anything you plan on regularly doing).
2. Assuming you're referring to the Telemediengesetz, there's a second law (Medienstaatsvertrag) which mandates an imprint for anything that's not strictly for "personal or family purposes". Depending on who you ask, those two terms also require a rather narrow reading, so anything beyond a strictly private family diary (careful not to make references to any outside persons or businesses, though, because those entities will then have a legal interest in being able to identify you in case you malign them!) or family pictures or your private Dropbox replacement (ideally all the above should be password-protected and therefore not accessible by the general public anyway) might again already be in a grey area.
2b. Additionally, blogs can enter another grey area where depending on what and how you're blogging about, they might be classified as a journalistic service offering and therefore require an extended imprint, too.
