$> cat infile
bla bla bla bla bla +
blob blob +
blip blip blip
would become
bla bla bla bla bla blip blip blip
$> sed -e :a -e '/\+$/ {N; s/\n/ /; ta}' infile
bla bla bla bla bla + blob blob + blip blip blip
would become
bla bla bla bla bla blip blip blip