May I ask which Beta Blocker you are taking? I started briefly with Atenolol, but I am now taking Toprol XL.
Since Inappropriate Sinus Tachycardia (IST) is a sinus rhythm (normal heart rhythm), it doesn't usually require treatment. This only means that your heart rate is faster than expected, thus the term inappropriate. Episodes of IST can be isolated, or they can be indicators of other disorders.
Some doctors have started to consider chronic IST to be a separate form of autonomic disorder/dysautonomia. This is on the same spectrum as disorders like POTS (Postural Othostatic Tachycardia Syndrome), NS/NMS (Neurocardiogenic Syncope/Neurally Mediated Syncope), PAF (Pure Autonomic Failure), etc. Episodes of IST are also commonly experienced with stress, and anxiety disorders.
As Leslie613 already pointed out, IST in itself is not at all dangerous. However, I know a great deal less about high blood pressure. Also alike to Leslie613, I have severely low blood pressure. However, Beta Blockers should be helpful in treating both high blood pressure, and IST.
I hope this is helpful, and also that you will be feeling better soon.
Inappropriate sinus tachycardia doesn't have a "cure" per se, but that doesn't mean it's a lifelong disease. Usually it comes and goes over time. First time I had it was two years or so ago, and then I've have it a few times since then, including this morning when I woke up. There are drugs that can treat the symptoms, like beta-blockers and calcium channel blockers. I'm on beta-blockers for neurocardiogenic syncope and the meds help for the IST too. Even though it sucks and often very worrisome, don't worry, it's not life-threatening. However, if you have other conditions you might want to check with your doctor.
I don't know about high blood pressure (I have the opposite problem-too low!) so I'll leave that up to someone far more knowledgeable than myself.