Over the years, the music world has seen its share of over-70 singers who kept performing even though they didn't have much of a voice left: Peggy Lee and Frank Sinatra are among the names that come to mind. But when B.B. King entered his 70s, the veteran blues singer/guitarist could still belt it out with confidence, and he does exactly that on Makin' Love Is Good for You, which was recorded when King was 74…