Not a lawyer, but as I understand it any publicly accessible publication would suffice. Actually I think any sort of open demonstration of the idea that others

Not a lawyer, but as I understand it any publicly accessible publication would suffice. Actually I think any sort of open demonstration of the idea that others can learn from is good enough, but I have no idea how edge cases are handled or precisely how open it has to be. The sibling comment mentions a good point about the falsifiability of time stamps. If you're particularly worried about being challenged, you might point the Internet Archive at your blog or keep your blog content in a repo that you push to GitHub. For the truly paranoid, consult an attorney.