I've been using Google Fit to track my blood pressure over time. I've been measuring every other day. The charts really are helpful.

The biggest correlation with my blood pressure is how regular my sleep is. Once I started militantly waking and sleeping on a set schedule, the numbers came down. They seem to spike for a few days if my sleep gets messed up badly somehow.

I also have been trying to standardize my measurement: same time of day, after sitting quietly alone for five minutes.