I use a fudge recipe that uses sweetened condensed milk and semi-sweet chocolate chips. All that I have tight now is 70% bittersweet chocolat

Similar questions have recently come up. You might search the hotline for a more prolonged discussion, but the short version is that using bittersweet chocolate is just fine. The two terms are often used interchangeably or imprecisely anyway. The end product may taste different than it would with another chocolate, but it should be just as good.