Is it possible to use my partially published previous work as a bachelor thesis? Some time ago I put up a question on the SE network. I ended up writing an extensive answer myself and published it in the same question and on my blog. Later, I put a lot more thought into the topic and found additional results, which I did not publish yet.

I have since considered if that question might be usable as a topic for a bachelor thesis.

Disregarding the fact that I would need to convince a professor to accept an existing topic, does the publication of the question and some results in the past prevent this?
Posting on SE or on your blog does not count as publishing in the traditional sense (the work has not yet appeared in a scientific journal or conference proceedings). This means that it is unlikely to be a problem. Just make sure you discuss it with your potential supervisor.