That is my feeling as well. Eastman & Laird signed ALOT of the initial 3,000 print run on the inside cover, though there are ovisouly a good amount that have not been signed. From what I have seen, few have been signed on the cover unless a) its a sig series or b) it was signed recently with the last few years.
I dont know if thats why they still get blue though. Maybe they are still knocking down the grade a little? I don't think the 1 9.8 and 5 9.6s are signed so maybe there is a max grade this book gets if signed (most of the signed blue have been 9.0 and below I've seen)?
As far as I know, if a book has a blue label with an interior signature, it has taken a hit in grade. You can do the same with sigs on the cover but I think those will take a much bigger hit for the universal label.