FOR decades she has been the quintessential homemaker. She sends her husband off to work with a kiss and a smile. She serves him dinner when he returns and soothes his ruffled ego when his boss gets angry. She politely vacuums around him as he reads the paper in his easy chair and tries not to disturb him when he naps on the sofa. In addition, she is a devoted mother to the couple's teenage son and daughter.But times change and families change - even the Bumstead family, America's favorite comic-strip version of traditional middle-class domesticity. Beginning last week, Blondie untied her frilly apron and got a job, becoming one more highly visible example of changing family forms. Her midlife career, says Dean Young, who now draws the comic strip invented by his late father, Chic Young, "is the biggest thing to happen to Blondie since the birth of Cookie Bumstead back in '42." Mr. Young cautions against reading too much significance into the change. Noting that he is a cartoonist, not a sociologist, he says he hopes the Bumsteads' new dual-career status will simply give him "more material" and "more gags." Indeed, potential scenarios abound: Will Dagwood help with the housework? Will Cookie and Alexander take over some of the shopping and cooking? Will Dagwood's friend Herb Woodley start urging his wife to bring home a paycheck too? And since Dagwood is no longer the sole breadwinner, might he decide to take early retirement, ending his stormy relationship with Mr. Dithers? Blondie's skills as a homemaker will undoubtedly make her a valuable employee - hard-working, cheerful, efficient. As she begins her new job, Blondie the late-blooming career woman promises to be a charming addition to the funnies. At the same time, Blondie the faithful homemaker will be missed - another symbolic entry on the growing list of vanishing American icons.