France Yvert 14Aa Vs. 14Ab (Is There Really A Difference?)

I have a few hundred 20c Imperforate Napoleons (Scott 15 Die 1) (Yvert 14A). The Y&T catalog lists color variations as follows:

14A bleu (blue)
14Aa bleu fonce (dark blue)
14Ab bleu noir (blue black)
14Af bleu laiteux (milky blue)

I'm disregarding the paper color variations for the purpose of this discussion. I am finding it really difficult to differentiate 14Aa from 14Ab. Internet searches on assorted auction sites (including Siegel) did not help. I've seen stamps identified as 14Aa that are darker than 14Ab. I've also seen the opposite. It seems to me that no one really can tell the difference.

Stamp color can fade over time. Darker colors can also be the result of a freshly inked plate. It doesn't have to be because of a different ink.

Below is a scan of three stamps. The left is 14A. (I don't think it is 14Af). The middle could be 14Aa and the right could be 14Ab. Is there really a blue black variety? It commands a premium price, so someone is paying extra for it. Thoughts?

The third stamp just looks dark blue to me. The illustration in the Yvert Specialised, which I'll try to post tomorrow, shows a quite distinctive shade for the black-blue.

Here we are. This isn't an easy book to scan, and the colours are not perfect!

They all look grayish black.

Yes - the differences between many of the images aren't that obvious in the book, but much worse via the scan. Désolé.