What would happen if I took a diet pill containing caffeine anhydrous with blood pressure tablets?

Taking caffeine anhydrous 50mg will most likely increase your blood pressure. Here's an explanation of caffeine anhydrous:

"The difference has to do with the amount of water associated with the caffeine molecule. The term "caffeine" can refer to any form of caffeine, regardless of its water content. “Caffeine anhydrous" has essentially no water—just 0.5%. For this reason, logic and chemistry suggest that anhydrous caffeine is more potent. The anhydrous form could also be more potent because, according to some evidence, the body may absorb it more readily. For all intents and purposes, however, caffeine and caffeine anhydrous are synonymous." Found at HPRC-online.org.

Use the following link and look under side effects "For the Consumer" and then "For Healthcare Professionals," specifically under Cardiovascular and Other:

https://www.drugs.com/sfx/caffeine-side-effects.html

To tighten up your tummy, you may want to do a search for "tummy tightening" or "tummy exercises".