In terms of medical research, no substantive reasons exist to "cleanse" the colon unless you are going to do so before a colonoscopy.
In fact, colon cleansing is detrimental to overall health because it can effect the ratios of good bacteria in your colon, can cause electrolyte imbalances in your blood, and can even lead to perforations in your colon which can cause bad abdominal infections.
I would strongly recommend that you do not undergo a procedure that is relatively risky and absolutely not based in science.
And besides, if you are worried about blood pressure medication, there are many lifestyle modifications you can try to lower your BP before going to drugs (of which there are many, many different kinds).
I hope you will talk to your physician before you try any alternative therapies for blood pressure modification!
To your health,