An Etymological Dictionary of the German Language/Annotated/bläuen
bläuen, verb, ‘to beat, drub’; instinctively allied by Germans to blau (blau schlagen, ‘to beat black and blue’). It is based, however, on a strong verb, Middle High German bliuwen, Old High German bliuwan, ‘to beat’; compare the equivalent Anglo-Saxon *bleówan, whence English blow; Gothic bliggwan, ‘to beat’ (with an excrescent gg), for bliwan. The root seems to be blu, from bhlu-; it can hardly be related primitively to blau, nor is it possible to derive *bliwan from a root bhliw for bhligw from bhligh (compare Schnee, Niere), and to compare it with Latin flîgere.