For most people, weight loss is 80% nutrition, and just 20% exercise. And there is a fair bit of evidence out there that strength training is actually more effective than cardio for longer term weight loss. So regardless of your heart rate, you should be able to lose weight.
As far as I know, activity still burns calories, regardless of better blockers.
However, beta blockers do make your heart rate a pretty unreliable means of tracking exercise intensity. You may want to look at the Rate of Perceived Exertion as an alternative measure of exercise intensity at www.sparkpeople.com/resource/fitness_artic