A stamp collecting forum I haven't mentioned before is Stampboards.com. A thread I particularly enjoyed had the great title "Banging the drum for the Uglies" and refers to the stamps of the various Indian States.
According to tonymacg who started the thread: "I don't know who coined the term 'the Uglies' for the Indian States, but it goes back a long way. Probably a Penny Black fancier. Anyway, it refers to the group of Indian princely states that, at different times between 1864 and 1953, ran their own post offices, and issued their own stamps - in most cases, valid only for postage within the borders of the particular State."
There are a number of photos of appallingly ugly stamps and information about them. Unfortunately, many of these can't be found in the "Blue" International but that doesn't make them any less interesting to the Classic Era Stamp Collector.
Jal Cooper in his Stamps of India book gives a good explanation of the difference between Convention States and Feudatory States which can be summarized as: Convention states overprinted the stamps of British India while the Feudatory States came up with their own distinctive designs. As I've commented before, Scott provides spaces for some of the Feudatory States but only a compilation page for the Convention State overprints.
The Convention states were comprised of Chamba, Faridkot, Gwalior, Jind, Nabha and Patiala and all were signatories of postal conventions with the British Government of India. According to Cooper, "these stamps were mostly used for internal postal services in the States, but they had a franking value of carrying letters outside the State limits to any part of British India."
Cooper provides a handy list of Indian States with the years they issued stamps. Two of these don't fall chronologically into the time period of the Blue: Idar and Jasdan. These full list is:
Bahawalpur (not in Pakistan)
Faridkot (both Convention and Feudatory)
Idar (1941-1950, not in the "Blue")
Jasdan (1942-1950, not in the "Blue")
Jind (both Convention and Feudatory)
Las Bela (now in Pakistan)