I worked on this same issue for 2 days. I used customizer to try blur and aosp calendars and nothing would work. Either I didn't have any calendar app in the case of blur or I had the icon, but it would not open in the case of aosp.
This is what finally worked.
1. turned off google calendar sync under accounts
2. deleted any calendar apps that might be present under applications
3. deleted liberty customizer folder
4. deleted liberty customizer zip
5. opened settings, clicked on liberty customizer, downloaded and loaded customizer
6. ran customizer and installed blur calendar
7. turned google calendar sync back on
* still can't add events to the blur calendar, but at least I have a calendar which is more than I had the previous 2 days!
I have had the same issue with both calendars, asop won't work period but if install blur and restart my phone then it works and let's me add events but only through the app not any Widgets