1. The legal definition of what's "commercial" under that law and what's not isn't necessarily quite as straightforward as a layman might think. (For one, that law doesn't literally say "commercial" ("gewerbsmäßig"), but rather "geschäftsmäßig" ("business-like"), which apparently can be interpreted to not require an intent to make money, but instead to 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.