I have pdfs of all items listed, and have printed out all but Gladiators and Elementalism (which I haven't looked into too closely as they don't appeal to me personally). From what I can tell the only noticeable difference between black cover and "revised" is the cover and a watermark on the pages of the black cover version.
Vikings, Pirates, and Samurai are all good and useful guides to their respective settings.
Arms is good and useful, mostly for things that weren't included in Mythras core (I give precedence to Mythras over Arms when they conflict). Also I really like how Alchemy is presented in this book.
Monsters is also good and useful, has some things that Mythras lacks, and stats some monsters differently.
Blood Magic is good, I like it and draw from it when appropriate. As for balance, it fits within the wide, GM-adjusted parameters of magic in Mythras.
Note a human's Base Move in Legend is 8, in Mythras it is 6.