The rule of thumb according to most clinical professionals is that
if you have a tattoo you can only donate blood one year after getting it.
The reason for this is that introducing ink, metal, or any foreign material into your body affects your immune system and may expose you to harmful viruses which can affect your bloodstream and will not result in a successful donation, since the blood will be compromised.
My fiancé and I are both tattooed and have both donated blood. I’m O neg so I try to donate as often as possible.
The reason that blood banks won’t accept a donation if you have a tattoo is if you’ve been tattooed in the past 6 months. This is because blood borne diseases such as HIV have a window period of 6 months at least before they become detectable. While any reputable tattoo artist should have sterile equipment and should always use new needles, the blood bank has to be sure that they do not transplant contaminated blood. It’s the same reason they won’t accept blood from drug addicts or sex workers.
Of course they have no way of proving your tattoo or piercing is older than 6 months: they rely on donors being honest on their forms. If you’re thinking of getting a tattoo but want to donate, rather donate 3 weeks before getting your tattoo, and then you can again after 6 months.