Um, I hate to be the one to say it but concealer, even the Kat Von D kind, will NOT cover a tattoo. It will look like you have makeup on your arm (or leg or wherever you get it) and the tattoo still shows through, even if you are a professional makeup artist you will have trouble completely covering a tattoo and have it look normal. So don't go into this thinking "Oh, I will just hide it" because you won't.
Others have said it all - scratchers give bad tattoos, HIV, AIDS, hepatitis, other blood borne pathogens. It doesn't matter if he uses a new needle strait out of the package. There are parts of his gun that need an autoclave to clean, and if he's not in a shop, he doesn't have an autoclave. Your parents could get in trouble for endangering their child, your parents could track down and press charges against the artist, and what are you going to do while its healing? You can't put makeup on a raw tattoo. It takes at least 2 weeks if not 4 weeks to completely heal a tattoo.
This whole idea STINKS of a bad idea. Just wait until you are 18 and get the tattoo done at a legitimate shop where you aren't endangering anyone with blood borne pathogens and jail time.