An Etymological Dictionary of the German Language/bläuen
bläuen, vb., ‘to beat, drub’; instinctively allied by Germans to blau (blau schlagen, ‘to beat black and blue’). It is based, however, on a str. vb., MidHG. bliuwen, OHG. bliuwan, ‘to beat’; comp. the equiv. AS. *bleówan, whence E. blow; Goth. 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 (comp. Schnee, Niere), and to compare it with Lat. flîgere.