Boke (meaning blur) is from a combination of kanji 暈 (bo) and hiragana け (ke). You'll notice that "bokeh" doesn't look like usual romaji (Japanese romanization) spelling, since it would normally be transliterated as boke. It was spelled bokeh with the extra 'h' to avoid accidental pronunciation like poke.
[1] http://www.luminous-landscape.com/essays/bokeh.shtml