Niech Was (or Wam) Bóg błogosławi!

Is there any different nuance of meaning between using the dative or accusative case with błogoławić, or is it simply a matter or stryle?

not much difference really, the usage is the same, one must think long and hard to find the difference and the bottom line remains the same anyway.
one is "God bless you" (directly you) and the other (wam) more like "God bless your ways" or "may God grant you his blessings" - see, it's rather hard to even discern, so I suppose you may call it a style preference.