The Collaborative International Dictionary of English v.0.48:

Blissful \Bliss"ful\, a.
   Full of, characterized by, or causing, joy and felicity;
   happy in the highest degree. "Blissful solitude." --Milton.
   -- Bliss"ful*ly, adv. -- Bliss"ful*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

blissfulness
    n 1: a state of extreme happiness [syn: bliss, blissfulness,
         cloud nine, seventh heaven, walking on air]