I should probably add a variable for caption text (as I already have variables for font and alpha), but the easiest for now would be to do something like
global.gameframe_caption_draw_text_orig = gameframe_caption_draw_text;
gameframe_caption_draw_text = function(_x, _y, _w, _h) {
draw_set_color(c_black);
global.gameframe_caption_draw_text_orig(_x, _y, _w, _h);
}