It may depend on what else in is the chocolate. Some additive maybe. There is some magnesium in dark chocolate, but magnesium is used for Erythromelgia treatments for some patients. I wouldn't expect the small amount of magnesium in dark chocolate to be much of a trigger. If your dark was high quality it is not treated with alkali during processing. This is supposed to spare the beneficial antioxidants in it.
Chocolate, aged cheeses and red wine may dilate blood vessels and have been linked to migraine triggers too. (this is less thought so today, but some doctors still believe it)

But there are people who are intolerant of chocolate, and the dark has a higher % of cocoa, than milk chocolate. Milk chocolate is really just sugar and fat with very little cocoa.

I get burning from MSG and potatoes! Each person is different.

Claritin is really a very weak antihistamine (some studies show it no better than placebo).. You might try Zyrtec if you are feeling experimental. The Benadryl never did much for my feet when I used to use it regularly. So alot depends on the person, and what is triggering the PN.