Recommendation :)

I'm looking for an anime series similar to Paranoia Agent, Ergo Proxy and Blood+ (gonna watch this soon). Typically anime which has a serious tone to it.

serious tone?

seriel experiments lain b4everything-else (if you are tough enough to watch it AND bain enough to follow and understand it) (but dont expect violence in this)

also:
kara no kyoukai
shigofumi
mouryou no haku
Kurokami
baccano (more or less, sometimes more sometimes less serious, all in all: awesome)

Both older so you may already know, but Witch hunter Robin and Noir seem to fit.
If it has to be harsher try Monster