Here's a discussion on that:
http://projetbabel.org/forum/viewtopic.php?t=6429
Can't say I followed it all, but the English "bless" is from "consecrating with blood", so you can imagine taking the "blood" emphasis instead.
[Origin: bef. 950; ME blessen, OE blétsian, blédsian to consecrate, orig. with blood, earlier *blōdisōian (blōd blood + -isō- derivational suffix + -ian v. suffix)]