On the subject of singing, the line between cabaret performance and jazz vocalizing has always appeared a little blurry; just consider how many people think of jazz pioneer Billie Holiday as a "nightclub singer," and how many others lump someone like Andrea Marcovicci in among the jazz chirps. But Jackie Allen does more than blur the line. She exploits the middle ground between these two forms to arrive at a quietly spectacular style--romantic yet unsentimental, musically sophisticated but instantly accessible--and in the process reveals her mastery of both jazz and cabaret idioms. At first hearing, Allen's voice has an almost wan quality, with a touch of honey--making her steel-trap swing and reserves of expressive power all the more surprising. (Luckily, she reserves some of that energy for her songwriting; her terrific debut album, Never Let Me Go, contains three distinctive originals.) A native Milwaukeean, Allen comes from a family in which everyone played music; not until she moved to Chicago and began cultivating the cabaret audience did she realize she "didn't have to scat all the time, to blow like a horn player on every song. I began realize that I don't have to the instrumentalists." Now she proves wisdom of that insight to the rest of us. Allen appears on a triple bill with the Bradley Williams Trio and the Jeff Stitely Quartet Friday, the first night of this weekend's Lake Shore Jazz Festival, celebrating the new label's first releases (Saturday night pairs the Marlene Rosenberg Quartet with fusioneer Byron Febbs). Friday, 10 PM, Bop Shop, 1807 W. Division; 235-3232. She's also appearing Saturday, 8 PM, at the Moosehead Bar & Grill, 240 E. Ontario; 649-9113.