I think a lot of that "____ is a Miracle Food!" stuff is overblown. Food isn't poison, it isn't medicine, it's just...food. Fortunately we live in a golden age of rationality and Google, so it's possible to look directly at nutrition information on a prospective snack. I find the stuff I want to eat by choosing what I like first, then looking it up and seeing how much of it I should have if I want to continue to lose weight. If I see nutritional info that I don't like (who knew persimmons were tasty little calorie-bombs?), then I just eat less of that thing.
I'm convinced grapefruit is on a lot of diet menus as punishment for having gotten fat in the first place.
Actually, it purportedly has enzymes that make it a "fat-burner," but there's zero evidence that they work that way inside the human body. Most grapefruit diets work because filling your belly with relatively low-calorie grapefruit means you aren't able to fill it with high-calorie foods.
On the other hand, eating most "miracle food" likely won't do any harm. From the "Can't Hurt, Might Help" standpoint, choosing grapefruit (or oat bran or acai berry juice or whatever the latest must-have food is) occasionally is fine. If you want to optimize your health, a good rule of thumb is to "eat the rainbow"--pick fruits and vegetables of a wide color range and you'll probably get a broader range of nutrients as well.
Lucky Charms, by the way, do not count as "eating the rainbow," unfortunately.
Non-starchy vegetables tend to be a lot more weight-loss-friendly than the starchy ones like potatoes and corn. You can eat piles of them for very little calorie cost. That doesn't make the high-carb ones bad, but they are more calorie-dense. So are most fruits (vile grapefruit is an exception; it's low-calorie because it's so nasty that even calories flee from that acrid horror).
As for the "negative calorie food" thing, it is (sadly) a myth. Even celery has calories, and they are indeed positive calories; the chewing and digestion of celery doesn't burn more calories than the celery itself contains.
Congratulations on your 40-pound loss so far, by the way! That is outstanding and I can't wait to get to where you are.