The apps AGR Blood Pressure Log and Blood Pressure for Health does not have any access to your data or to personally-identifiable information.
Your data stays where you put it
You have sole control of your data. It exists only on your device, in your backups (if you back up your app data using iTunes or iCloud backup) and at whichever locations you have manually transferred it to using the app's export features.
For this reason you should make copies of your data using the export feature to protect from data loss in case of accidental deletion of the app or loss of your device.
Sharing data with Health Kit
If you want Blood Pressure for Health can share data with app Health. This requires your explicit authorization. If you do, a copy of the samples is automatically sent to the app Health.
You can revoke that authorization anytime using the options that enables app Health.