There are many different mistletoe species and the chemical properties change with the influence of the host tree.

http://www.cdfa.ca.gov/plant/ipc/weedinfo/viscum.htm

Much of the literature I see with regards to the use of mistletoe for the treatment of hypertension is outside of the USA.

Here are some helpful links:
http://www.cancer.gov/cancertopics/pdq/cam/mistletoe/patient/Page2#Section_16

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3555592/

In summary, I would proceed with caution.

Chervil's right. From my experience, mistletoe helps in only a small percentage of hypertension cases because it works much on the Heart network. However, most of hypertension problems involve congestion/stagnation/heat starting in the Liver network and spreading to affect other networks including Kidney and Spleen (sometimes). This is where having a good herbalist who can analyze your personal conditions comes in handy.
I rarely give a simple (1 herb regimen) for chronic problems. The idea in herbology is to feed all the affected networks and their relationship to one another.
Be safe. For the most affective tincture, it's best to make the tincture/extract from fresh mistletoe BEFORE it flowers.

HerbDoctor