Abdomen,
I doubt they would even ask you if you have one so I wouldn't tell them, it would be stated in the terms and conditions that they are not to be visible. I recently had a flight with a carrier here in Australia however where the CM was wearing the short sleaved polo and every time he stretched or leant over his arm tattoo was clearly visible, not really a good look. Although you can do your best to try and hide it some uniform designs just don't cover it all so that is a consideration. Many FA's have tats, me included, companies know this. If it's on your shoulder though I would just forget it's there and go in for an interview with confidence.
I remember doing wet drills once when we all have to get into our swimming gear, we were all worried about this, turned out even the trainers had them!!