Looks to me like the cv boot took a dump, and the previous owner never bothered to clean it up, so mud/dirt/rock/etc. just got stuck with the grease. Over time that crap just accumulates and turns into this crazy tar like substance (don't ask me how I know).
I second (or however many times it's been said) the wire wheel method to remove it all. Wear a mask when you do it though. If you really feel froggy, and plan on re-finishing the entire wheel, just go get some aircraft paint stripper at the local Checker/Advanced/Etc. shop and give it a whirl. That stuff takes ANYTHING off.