Goodduck, sounds like a good session, and you were lucky to get to flag.
In the UK a marshal wouldn't be allowed to flag without many years experience and grading up the ladder, so to get to do it on your first time, certainly a baptism of fire. Hopefully you had a good day.
In European racing, a black flag is only shown on the startline to inform a driver he is being removed from the session/race. And a session stop is a Red shown on each post, cars to return to pits/grid as per rules.
Blue flags should not be used when it is 2 cars fighting for position. At that time it is for the car to pass/defend within the rules, and should never be shown at the same time a yellow is out as they mean opposite things.
As said, some series have slightly differing rules as to what can be shown when, but normally the rules are fairly stable per country/organiser, and road or oval.
You need your wits about you, not moving your concentration during the session, especially if you're on the yellow.
Good for you giving it a go, I hope you enjoyed Marshalling and will be returning on another weekend