WHAT CASE DOES BŁOGOSŁAWIĆ TAKE?

What is the difference (if any) between "błogosławię Wam" and "błogosławię Was",
ie dative or accusative??

'To bless' plus the genitive case or 'to give one's blessing to someone or something' plus the dative case.

Merged: błogosławić komu/czemu or kogo/co?

The verb błogosławić may be encountered governing either the accusative or dative case. Is there some faint nuance of difference between the two or are these stylistic options?

błogosławię (co?) ten dom
błogosławię (czemu?) temu domowi

This is a faint nuance. Your question is difficult actually.
Both are correct however the second one is rarely used today and having its derivation in oldfashioned Polish language.

Do you agree that oftentimes something slightly archaic has a loftier or more nobler ring to it?

No, not loftier nor noble, just in your head polonius. and it's not co/kogo but cos'/kogos'.