If it's just a little bit, and it's in the mucus, it's not as big a deal as it may sound. Yes, you're still coughing up blood. But it kind of happens to us. I've coughed up streaks (that's a little bit of blood in the mucus you regularly bring up) for something like 2 years now. Just this past Feburary, I coughed up pure blood for the first time. And even that wasn't a huge deal. It was all blood, but it was only like 2 teaspoons. I emailed my doc right away (my fiance was worried and I promised him I would) and the doc said "As long as it was a little bit, and it doesn't continue, no need to worry." I had just started nebulized Colistin, and asked if I could stay on it, even though it might be the cause of the blood. Doc had the same attitude "As long as it doesn't continue, you can keep on the Colistin, I know it's helping."
Basically, I'd tell your doc, but don't freak out too hard. Plus, I may be wrong, but the hypertonic saline is pretty abrasive. I just started it last month, and it works real well, but it is rough. It may be causing the blood. As long as it doesn't continue too often, and doesn't increase, there's no reason you can't keep using the saline. I'm still on the Colistin even though it made me cough up blood. *shrug* That's a lot of talking. And I'm no doc, and I can't see your lungs. But in my opinion (from what you told us), there's no reason you can't keep using the saline. Just be honest with your doc, so she has all the info, because that'll properly equip her to make the best decision.