Several stamps in the Blue fall into the category of stamps that were once in the catalog but have been delisted. I think one can argue that if they were in the catalog at the time the album was issued, then, of course, they belong there and the compulsive Blue collector needs to fill those spaces. (In every case I've discovered so far, delisted stamps like these are still readily available.) But does it become a "Blue-per" for those editions of the album that Scott has made other changes to, but "neglected" to remove stamps it no longer recognizes as officially issued?
The first case in point are the Armenian stamps printed in 1920 that are still in the International and occupy the entire second line for that country. According to my 1943 Scott Catalog, the stamps "were printed in Paris. A large quantity was lost on the way to Armenia. Before the balance was delivered the National Government had been driven out by the Bolsheviki and the use of the stamps was prohibited." The Scott Classics Catalogue prices the set at $5 and notes that you can find some of the stamps fiscally used and with specimen overprints and imperforate. Adding to the fun, the set of ten has been reprinted. Scott says that the colors of the reprints are brighter. Interestingly, these stamps are also in Yvert&Tellier (I don't know about other catalogs) as numbers 94-101 without any notes questioning their authenticity and valued at 5.50 Euro.
There is a little more information on the Web about these stamps that you can find through Google. You can also see them in the Antonius Ra Collection with a notation that they weren't issued.
So should Scott have just dropped these stamps and left a blank at row at the bottom of the page? Well no, they could have given collectors spaces for Scott 300-309, the set of ten stamps from 1922 that catalogs a whopping $6.95. Now there's something that even I can afford.