When a noun, pronoun, adjective, or verb has a long vowel or diphthong in the accented syllable and its stem ends in a single -l, -n, or -s, the -r (or the elder r- or z-variant ʀ) in an ending is assimilated [REF].
Blása, to blow, has blæss for "you blow" instead of *blæsr (*blæsʀ) [REF].