I tend to be skeptical about that account. I find his account rather biased. Not to say he isn't useful, nor that there weren't important blots at Uppsala(there almost certainly were), but his particular account makes me skeptical. Now we do have an account of something that could be regarded as human sacrifice and that is Ibn Fadlan's account of a funeral of the viking Rus which has a slave girl killed.
I will eventually be altering the Blot event the norse have atm so it doesn't have human sacrifice, still working on the specifics of that though.
Anyways, I do have my plan for what I hope to get out by the end of the week and that are offerings. Currently the plan is that everyone gets a general sacrifice and a religious specific sacficie decisions. The general religious sacrifice is a simple thing click it and you get a little bonus to piety and same religion opinion.
The religious specific sacrifice though is a little event chain, where you pick a deity you wish to give sacrifices to for blessing, how big of a sacrifice you want it to be, then the sacrifice itself, followed by the priests telling you if gods have blessed you or not. If so, you get a small bonus relating to either war, trade, fertility, peace, or research, depending on whom you offered to. If the priests say no blessing, then you just get the performed sacrifice modifier from the general sacrifice decision.
Currently you can only do them if you are of that particular faith, but I want to have it so if you have a province of that faith and are pagan you can do these sacrifices. It doesn't really change much considering the bonuses are all the same, but still just a little flavor thing.
Here is the currently deity and what bonuses you get for doing offerings to them.
Slavic: Perun(war), Dazbog(trade), Mokosh(fertility), Radegast(peace), Veles(knowledge)
Norse: Thor(war), Njord(trade), Freyr(fertility), Tyr(peace), Odin(knowledge)
Finnic: Perkele(war), Mielikki(trade), Akka(fertility), Haltija(peace), Vainamoinen(knowledge)
Baltic: Perkunas(war), Mara(trade), Saule(fertility), Gabija(peace), Veles(knowledge)