What I've been told about Stretch marks is...if you're prone to getting them, no products will make your body escape them. They do lighten with time...I have some too, but after 4 years, they are my skin color again. (Not sure when they turned back to the normal color, but I know that right away, they were dark purple like you described.) Apparenty, once you have the stretch marks, they're really no way to rid yourself of them. The skin doesn't just grow back together. But, you can tighten the skin by doing major Ab excersizes to reduce the flab.
I just realized I didn't comment on the tanning thing. I've been told by doctors that you should use extra sunscreen on new scars/incisions that will be exposed to sun or cover them up completely or the new skin will turn purple as the sun will burn the new skin. Don't know if stretch marks are the same as scars, but could be theoretically. I do know that you can't use an OTC sunscreen in a tanning booth (I used to work at a salon and worked at another company that sold products for tanning beds, like beds, bulbs, lotions, etc.) The only sunscreen is to not go in a booth or cover the area you don't want tanned.