Are the CMS.BlogPost fields marked as searchable on the Page Type?
Does this happen in the Search Preview tab as well? Or can you see them there?
I'd also try adding in CMS.BlogMonth and CMS.Blog to see if that changes the behavior.
Also you can use the third party tool LUKE to look at the contents of the actual Search Index itself to make the data is in the index or not (and remove any issues with the Kentico site displaying it).