บ is used in Thai as a negative particle in poetic contexts, I think. For example, in the บทสวดมนต์ สรภัญญะ : ตัดมูลเกลสมาร, บ มิหม่นมิหมองมัว "Who have cut the roots of defilements and are not sorrowful, not dark and gloomy." Translation source: http://paultrafford.blogspot.ca/2013/03/a-buddha-prayer-song-translated.html?m=1http://paultrafford.blogspot.com/2013/03/a-buddha-prayer-song-translated.html?m=1