Is it possible that being underweight, but having a high amount of body fat, could be a cause for high blood pressure?

Sometimes: There are many problems associated with elevation in body fat, including the risk for metabolic syndrome for which hypertension is part of the picture. This story is evolving, but the inflammation, especially in visceral fat may be associated with all elements of that syndrome. There are other reasons for hypertension not related to visceral or body fat, so discuss with a doctor your risks.