I'm sure you're just fine, as it appears someone said their HE-6 doesn't have this unique finish. From the time I owned them in 2011 and now, from early 2013, I have always had this finish with a purple hue. I have never seen it on the 5LE or 500 though.
I suppose to get to the bottom of this is contact HifiMan. If they state it never had a pure black finish without this hue, then I would be concerned. Where did you purchase yours? I bought mine from Audio Advisor, which is an authorized dealer.
My HE-6 looks more like dark indigo than purple to me, is it supposed to be purple?