My 4th great-grandmother was Elizabeth Bly (1780-1855). She married a John Grow. She was the daughter of Jacob Bly (d. Shenandoah Co., VA) and Catherine Humble (d. Frederick Co., VA). Jacob was the s/o Philip Bly/Bley and Maria Magdalena Unknown. Both died in Shenandoah Co., VA. The family also spent some time in Lebanon, Lancaster Co., PA.
I haven't gone back further, but some researchers states the line goes back to a Nicholas Bley of Hornbach, Zwiebruchen, Germany.