That happens sometimes. It's nothing to worry about.
How much blood was there ? If a little it was a capillary no worries. Remember pinch your skin insert needle let go of skin then inject. Most of the time there were a few drops of blood when and always a red spot it was never blue.
also straight in straight out. give 15 sec or so before pulling out. i never had blood, bruising or any marks. good luck.
As a wise soul said here once, "If this was rocket science or easily screwed up, they wouldn't allow us to do it ourselves."