Just for information for those interested.
I have a Garmin Index BPM registered to my account. Since registering the Index BPM on my account I have the Blood Pressure menu item under 'Health Stats' in Garmin Connect Web and Garmin Connect iOS. I also have a 'add entry' option now that lets me manually add a Blood Pressure/Pulse entry.
While a little tedious, I was therefore able to add my historical blood pressure readings from earlier this year that I had taken with my Omron IA2 blood pressure monitor.
It would be nice if there was some sort of import feature to allow users to create the historical record of blood pressure readings.