I changed it to each post, and changed how you got the value for post. This has been edited to address your question in the comment: When the bottom of the post goes below the scrollTop then it will lose the seventy class.
$(".post").each(function () {
var postTop = $(this).offset().top - 20,
postBottom = $(this).height() + postTop - 100; // -100 is just to show it work, remove.
if (scroll >= postTop &&
scroll <= postBottom) {
$(this).addClass("seventy");
} else{
$(this).removeClass("seventy");
return false; // more efficient, but might not work.
}
});