I've read that benadryl (diphenhydramine) can both raise and lower blood pressure - which is correct? i take 50mg at night to help fall asleep.

Neither: Benadryl (diphenhydramine) has no direct effect on BP and is one of the safest medicines available and fine to use as you're using it for sleep. It's given to the very elderly and the very young safely. Remember that you can purchase it inexpensively as the generic: Diphenhydramine which works just fine also.

Sleep Disorder: If you are taking medication to fall asleep at night you probably have a sleep related breathing disorder. Sleep problems can cause high blood pressure in addition to many other problems including higher incidence of stroke, heart attack, prediabetes, etc. If you are experiencing poor sleep and/or on a regular basis see you md about a sleep evaluation.