Is it possible to publish a paper on a topic about which you already wrote an online article?
This is perfectly acceptable.  It is, in fact, fairly common to "upgrade" a white-paper, technical note, or other such non-peer-reviewed publication into a peer-reviewed publication.  Two important caveats, however:


The appropriate form for a peer-reviewed publication is generally very different than for a blog post or similar informal format.  You will likely need to almost entirely rewrite the publication in any case.
If the other material is durably available or has been circulated in academic circles previously, however, you should acknowledge this fact and reference the prior publications.  This may be in the form of a citation, or in the cover letter of the article, or both.