The answer to this question depends on the context in which the question is being asked. In some cases, blog posts may be considered formal, while in other cases they may not be.
Generally speaking, if the content of a blog post is meant to be taken seriously by other professionals in a particular field, then it may be considered formal. For example, a blog post about presenting at a conference may be considered formal because it is meant to be used as educational material by other professionals.
On the other hand, if a blog post is meant to be entertaining and fun, it may not be considered formal. For example, a blog post about how to make pizza using a new recipe might not be considered formal because it is meant to be enjoyed by readers.