FYI for female applicants of Emirates and Etihad- female crew do NOT get the option of long sleeves on board. Yes there is a cardigan but you cannot (and would be crazy to) wear it in summer.
Therefore, if you are going to 'hide' anything, it would come out in the medical in Dubai, and if severe enough, then yes if undeclared it could send you home.
Don't listen to friends who have "barely noticeable" scars- the original question is regarding a port wine birthmark covering a whole arm- not exactly "barely noticeable"....