So I run a pathfinder game that is set 20 years before the birth of the Nerevarine... Without much background on the next location my players want to go.

See, they will soon be headed into Cyrodill in an attempt to warn Uriel Septim about a plot to awaken the Dwemer, something they learned while clearing out "bandits" from a Dwemer ruin in Skyrim. Afterwards, the de-facto party leader, an Argonian thief has plans to return home and gather up an army, something the rest of e party agreed with...

I'm currently at a loss, because it's a good plan, but know next to nothing about Blackmarsh, beyond that it svcks to go to if you aren't an Argonian, and there are moving trees... Where can I go to learn more beyond what I've read on UESP?

http://www.imperial-library.info/

I would take a look at the footnotes for The Infernal City and Lord of Souls since they both gave us more lore on the Saxhleel and Argonia.

http://www.imperial-library.info/. Read everything, The http://www.imperial-library.info/content/pocket-guide-empire-and-its-environs-first-edition http://www.imperial-library.info/content/pocket-guide-empire-and-its-environs-third-edition and http://www.imperial-library.info/content/argonian-account would be a good choice if you haven't read them already.

I've been meaning to pick up Infernal City, it has lore info on Blackmarsh? That's great!

It actually starts in Black Marsh. Actually, the main characters lived in Black Marsh...

Then that's a definite purchase.

Well, two of the four main characters live in Black Marsh, both of whom are good friends, and one of them is a Saxhleel (argonian).