I agree that terms like "dark web" and "deep web" have been used so much (and often interchangeably), that they've mostly lost their meaning. That being said, I've always used "dark web" in reference to things like hidden services, i2p, etc, and I've used "deep web" to mean unindexed/inaccessible content.
Imho the "dark web" means those parts of the web not publicly visible, ie not indexed by search engines and/or places not accessible to the public. Most hidden services are indexed and are publicly accessible ... so they are not by definition dark.