I had this problem when trying to burn cards using a bic style lighter, try using a zippo style lighter, and use the very tip of the flame to burn the card.
Blackening cards is a little difficult. You can't hold the flame very close at all and some warping may occur. You will then have to pull the flame off and put the cards back in the deck, put them in the box and then put heavy books on top of it. In the morning they will be flat again. If you allow the bubbling to occur you've already heated it too much.
I make these types of cards with a hold through the middle with singed marks for a cowboy shoot a card effect in my kids b-day shows.