I've heard about Gymnema Sylvestre, which may possibly help in the treatment of diabetes, though it should probably be taken under the advice of your physician, especially given your health history. It doesn't actually block sugar absorption, but it has been shown to improve blood sugar levels. So 'blocker' is not a good description. The calories will still count.
I've not heard that it changes the way sugar tastes, so they must be adding something to the supplement you were looking at. If you do decide to take it, please read the labels carefully and make sure there are no additional ingredients in the product which may be harmful or unnecessary. It's common practice for many weight loss supplement makers to include a long list of ingredients that they claim will help, but are often proven otherwise and may cause side effects.
A lot of our members get rid of sugar cravings by cutting them out for a week or two. This includes not just sugar, but many foods that are high in natural sugars, such as fruit. Phase 1 of the South Beach Diet is intended to kill sugar cravings and works for a lot of people, but not everyone.
Hope that helps