i wouldnt worry.. if its not visible in your interview and therefore in your potential uniform its not visible at all so if they ask you if you have visible tattoos you can say no.
there are plenty more obvious places to have tattoos that can cause a problem with work uniforms - and i know tons of tattooed FAs that hide it well.
If your work uniform is a white shirt.. just be concious that you may need to wear an undershirt as sometimes white cotton shirts are almost see through.