I'm pretty sure they're supposed to show 2 lines, even if its a faint negative. Our bodies produce LH even when we're not ovulating so its picking up on that. When we ovulate the levels can increase like 5x as much and that's what causes you to get the darker line and a positive OPK.
I haven't tried doing BBT but I know one of the apps I use called Ovulation Calendar allows you to track if you've had sex, taken a prenatal vitamin, BBT, what type of CM you have, what position your cervix is in (which I don't know how to tell), if you've gotten a positive OPK, etc. It apparently compiles all of that you tell you when your fertile window is. Its also nice to look at the calendar and see when you've ovulated and what days you've BD in relation to that.