It is not an official figure published by Naver. Outside analysis services estimate it from data such as visitor numbers, post rankings and how long the blog has been active, and show it in tiers such as "optimal" and "semi-optimal."
The blog index is only a reference, so you need to look at whether posts are written to match search intent and at actual rankings as well.
How MAKE Marketing uses it
MAKE Marketing refers to the indexes of top-ranking blogs to judge which keywords we can compete for, then confirms with actual rankings.