Fludrocortisone and Midodrine are both medications that can help raise blood pressure. Ask your physician if one of these medications would be right for your situation. People are not generally prescribed medicine for low blood pressure, as it's not often a cause for concern unless it presents with symptoms.
Why do the middle fingers lock up when experiencing pain in neck and shoulders?   One cause could be some kind of impingement (pressure or swelling that applies pressure) of the spinal cord at C7, one of the cervical segments of your spine.
Flexor tendons of the thumb and fingers pull the fingers into a fist. The tendons are enclosed in a synovial sheath. When this sheath becomes inflammed it is called trigger finger. Usually, tendons slide easily through the sheath as the finger moves. In the case of trigger finger, the synovial sheath is swollen. The tendon cannot move easily. This causes the finger to remain in a flexed (bent) position.