From my limited understanding (IANAL and so on) you don’t need to register a trademark to make it enforceable. If you can show a track record of using a name and logo by a project related to a particular product (software) before the alleged infringement, that trademark is enforceable. It may become unenforceable, if said project does not take any steps to protect its trademark. Protecting does not necessarily mean forbidding any 3rd party use, but e.g. some kind of quality control and/or delivery requirements (e.g. here both the bad documentation (Q&A) and making money off the trademark would apply). Of course actually enforcing a trademark is a different beast. Starts with someone caring enough to put in work into a policy, the “project stakeholders” (probably hard to define in itself) to agree to have a trademark and then enforcing it, i.e. going against infringements like the store listing here (if that is considered an infringement). Some resources:
https://google.github.io/opencasebook/trademarks/
http://modeltrademarkguidelines.org/index.php/Home:_Model_Trademark_Guidelines
https://fossmarks.org/post/
But again, probably all moot as my impression is that darktable is in the usual situation for such projects: People involved are motivated to create a technically excellent product, not dealing with paragraphs 
(Which - if that’s not obvious - I can very much relate to).