Blessay is a merger of the words Blog and Essay. It describes a longer form of blog posting that contains factual, researched information that is likely to bre presented in a coherent, educated fashion with supporting research, bibilographic details and so on.
In concept, a blessay is more that a simple spurt of verbiage (as many of my blog posts are), but a coherent train of thought exploring a concept in some detail. Probably less than a full prepared essay or paper, but an attempt to provide coherent and rational consideration of the topic in hand.
I believe that Stephen Fry was the inventor of this word, and you definitely take time to read his very fine, erudite blog.