Blog is more informal, personal, diary-style. Article, news site, analysis, is more formal, more professional. But sometimes, it is difficult to set the borderline. If you are an amateur but getting very good, you convert in a professional in some moment. You don't even notice it.
In another sense, professionals are living from this, amateurs, not. For the latter this is only a hobby or a second income source.
If your income doesn't depend on it, make sure you enjoy it. But that doesn't mean you shouldn't get slowly better.