Single blue blaze = side trail, go straight. I remember seeing these on the Appalachian Trail; it's kind of amusing to find them in our little neighborhood park. (There are no white blazes there. Zen koan: can there be a side trail, when there is no main trail?)
Well, it's good to know that, if I keep my wits about me and follow the signs, I won't die of exposure, lost and alone, and be eaten by the crows and feral cats, a hundred yards or more from the nearest road.