At a first glance the difference might not seem that big. Both are written works conveying a scientific topic. But there is a difference and it is not negligible.
Whether you already have a done piece that you want to publish or if you are just getting started, you should consider how you want it published. Here are described two different ways to publish.
A blog post can be written by in principle anyone. You can set up your own blog and start writing. If you do, you can benefit from having a Twitter-account to use for marketing your newly written contribution to public understanding of science.
All proof-reading is done by yourself and you also control all marketing. This can be an advantage or disadvantage depending on your skills and how big and relevant your network is.
An article is typically a part of a magazine owned by a publisher. This gives the advantage of having an editor that is en expert in the framework of your article, i.e. the structure, language and general layout.
It is more difficult to get your popular science article out as an article in a magazine than on your own blog. The advantage is that the magazine most likely will already have an established base of readers as well as a clear process for marketing and advertising. Additionally, the article might be printed in a physical issue and reach an audience that does not usually use online media for news.
A disadvantage can be that the article can be hidden behind a paywall or subscription fee, which can make some readers turn to other news sources.