When you grandmother was born,
Darlene was a much "younger" name than
Velma and most people her age would have much preferred it. I assume your grandmother was born in or near the 1940s. Back then
Darlene would have had a much "sultrier" image than
Velma. And when your grandmother was young,
Velma of course wasn't yet "retro", it was just "old". It's perfectly logical that someone your grandmother's age would hate being named
Velma. :)