I think you could get a tattoo and make it all about Dharma, but few people are willing to put in that level of effort. This is just my own opinion of course, Im no authority on anything but this is what I would do if I got a Dharma tattoo. Im considering getting one which is why Ive thought this all through.
First you should do it out of Bodhicitta, that you sincerely wish to attain supreme enlightenment in order to bring ultimate benefit to others.
Second you should find an artist who is a Dharma practitioner and will give you the tattoo out of Bodhicitta as well.
Thirdly the tattoo should be done on an auspicious day. If its going to take multiple sessions then they should all be done on these days only.
Fourthly the cost should be divided among these auspicious days so that each payment is an auspicious number. For example, $108 per session done over three (for the triple gem) or four (noble truths) sessions. Whatever the cost or number of sessions, it should all happen on auspicious days and be of auspicious numbers.
Fifth there is the money itself. A portion of it should be donated by the artist to a Dharma cause, or the artist should be willing to give a discount (like %10) or to forego their tip, and that this money will go towards a Dharma cause. Another possibility is a commitment to donate an amount equal to the cost of the tattoo for example, even if only a bit at a time as you are able. So if you spend $500 on a tattoo, you commit to give $500 to Dharma causes in the future.
Sixth you should begin each session with opening prayers and conclude with the dedication of merit, the artist should be willing to at least sit in on this if not participate.
Seventh you should recite a mantra or do a visualization or contemplation associated with Dharma while getting the tattoo. A tattoo of Manjushri for example is an obvious one, reciting OM A RA PA TSA NA DHIH while getting the tattoo done. If possible the artist should do this too, but if its distracting then there is no need since you dont want mistakes to be made! Furthermore if music is playing in the shop it should be Dharma songs/mantras/instruments and not just ordinary music like hip hop or heavy metal.
Eighth in regards to pain and blood, there are easily contemplations you can do for these. If you cannot maintain the view of equality and emptiness while getting tattooed, then you should think that since you can barely stand this pain now, so how will you stand the pain of the hell realms if you don't achieve enlightenment? Since that pain is much worse and lasts many times longer, you should contemplate in this way and consider the four thoughts that turn the mind from Samsara. As for blood there are two approaches you can take. One is that you can imagine that the loss of blood is the loss of negativity, and the receiving of ink is receiving the nectar of purification. At this time you could do Vajrasattva for example. Another method would be to see the blood as an offering to the beings of the lower realm, and the pain as self sacrifice for the benefit of these same beings who otherwise could not receive sustenance without your sacrifice, willingly offered out of love and compassion for them and the wish for them to attain liberation from their miserable state.
Ninth you should wish that every being who sees this tattoo will be inspired to practice Dharma and be liberated thereby.
And finally you should think that whatever the theme of the tattoo is, that you perfectly and completely receive its blessings and realize its ultimate meaning. That the accomplishment which it represents, whatever it might be, enters your mindstream fully and purifies all your negativities.
However most people wont go to this extent. Yet this is one example of how something ordinary and worldly can be turned into practice.
Most will get a tattoo because they think its cool, out of attachment to the idea. Possibly they will have a little faith that the tattoo will bring some blessing but because they put in so little effort, the result will also be little. Rather than doing mantra they will think ordinary things, rather than using the pain as practice they will just wish it would end. Rather than seeing it with pure view, its just very ordinary most of the time. Literally every time I see someone with Tibetan tattoos I find out they have little to no interest in Dharma, know next to nothing about Vajrayana, and barely understand the meaning of what they have. Some of them even have forgot what the meaning was or have no idea how to pronounce the word or what it says.
If on the other hand someone puts forth this much effort and really cares about the result, I see no reason why getting a Tattoo could not only be the cause for receiving blessings, but even possibly liberation itself. Its really on the practitioner to transform their actions and life into Dharma or not. Individual actions are neither good nor bad in themselves, its our intention and skillful means when performing them that will determine if it brings benefit to ourselves and others or not. The methods and ways of Vajrayana are inconceivably vast, being as numberless as there are phenomenal appearances in all of existence. Nothing is beyond its scope, but whether or not we transform our ordinary actions into practice depends upon us.