Most foods aren't " good " or " bad ". Some may cause individuals cravings, or they have allergies to them. Some are addictive to certain individuals. These are only " bad " for those people.
As a diabetic, bananas are " bad " for me. Sure, I can eat 1/2 of one in theory, but in practice even that spike my blood sugars over 200. For everyone else though, bananas aren't considered " bad ".
Foods have some good, and some bad, and you just want to get more good than bad in your diet. You can have a scoop of ice cream and have no issues, but eating a whole pint daily is bad for you. As long as you can control the quantity of these foods, then you won't have any problems.
If the chocolate can be eaten in small amounts, and has some positive effect on you, then go ahead and make it fit into your daily menu, if you want it. The problem with regular chocolate, is the added milk, and sugar.
The overall health is going to be based on the quality of the foods you eat, and the combination of them. So no one food is going to ruin the diet, and be " bad ". Maybe a dish/meal, but you can usually plan that also. Just stay in ranges, and avoid foods that you " can't live without , but otherwise, if an oz. of chocolate keeps you from going off plan, enjoy it.