rather than calling him selfish I would say probably very insecure. He has been built up for forever as sensei's inspiration but other than that he's not treated like a human. Im sure he's just scared that he can be replaced by any other person who wants a tat.
would go with that, i mean he said it at times that he's the inspiration and it did sound like self assurance backfire but if this was all then he would not do something as to have a physical relationship with the tattoo artist seeing how he understands something like this can throw off his position to the artist for that he might be objectifying himself to someone else , yes it's not that he's insecure, he wouldn't do it if he was and would just try to save his place at whatever cost...
ps: i know i said objectifying but i'm fine if this turns to a love story ^^
that metaphor could have been thrown at us less harshly, damn it, i'm not sure but blood in art is the core of the character, the uke's character was only under that selfish artist, until he got his hands on something like being cared for as an actual being *sure the artist gave him place but u can see when he saw anther costumer how self centered he was, he needs more, which i think is his true shape not the one we saw at the start being the doll for some artist*... thus his characterization should also start to shatter the way his previous believe that he wants to be the artist's living piece, seeing how selfish he actually is , even in the sex scene, he said something like he wants more *not very sure about the sex talk..*