Is an idea still 'novel' when already published in a blog post?
Whether it is novel enough to be published is up to the editor of a journal.
But you are in danger of plagiarizing unless you are very careful about citing the  original idea. The idea isn't yours, but theirs. The &quot;novelty&quot; isn't yours, but theirs. The fact that you noticed that it was something new isn't the same as creating something new.
But even if you avoid plagiarism, you might be accused of scooping the originators if they have plans for more formal publication. I suspect that if they have a paper in preparation and complain to an editor you will be in for some problems.
The best way, I think, is to contact the original author(s) and ask them about publishing, offering to help and working out some details about authorship.