I have Masters degree with Distinction and Student of the Year Prize in Corrosion of Engineering Materials from Imperial College London, first degree in Chemistry from University of Central Lancashire and also did Technology, Material Science and Engineering and Mathematics from the Open University.
In areas where tattooing is not as closely monitored, people are asked to wait for 12 months to give blood after getting a tattoo. This waiting period ensures that the donor is free of any potential blood-borne diseases which could have been passed on unsanitary equipment.
Yes, it is true that you cannot donate blood for till 6 months after being tattooed. This is because blood donated by such people carries the risk of being infected as it involves the use of needles. This is just an infection control measure. Caution is important to cut chances of infection.