You cannot plagiarize your own content.
You would be best to reference that you have previously published the article. List where you published and possibly in an introduction why you decided to republish and do so on Steemit.
Your own content produced is yours to share any place you wish.
Introducing the republished work and showing the locations of previous publications can save on a lot of drama when people search for similar content and find it published elsewhere. The mentioning why you republish on Steemit is mere respect for other users.
If you are the author and these earlier blog posts are your genuine work, then I would not consider it plagiarizing.