just fixed the same issue, my project was upgraded from 5.5x. The problem is on the page type. I don't know what actual problem. Even through I imported the same blog post page type from a fresh 8.2 copy to overwrite the existing page type. The issue cannot be solved.
What I did is export the project site full copy and import into a fresh installed copy. when you import, please use select only new or uncheck all blogs page types if you would like to overwrite other page types, which would use import the data to a new set of blog page types.
I think this is the safe method to keep all data and use fresh your Kentico CMS. After this, I would think the problem may be caused by the whole set of blog document / page types, which does not upgrade probably along with many versions upgrade. (5.5 > 5.5 sp > 6 > 7 > 8 > 8.1 > 8.2). You may also try to export CMS.Blog, CMS.BlogMonth and CMS.BlogPost from a fresh copy and import to project site to overwrite them. The data will remain the same if there is no fields different. However, you may need to fix the field validation for some fields (I got problem on Post Title field after overwrite the CMS.BlogPost page type, just remove the validation of that field, save, and apply the same validation again will fix the issue of field validation fail when save the page.).
** Remind that, always keep a full backup before any try and fix. Good luck!!