Most use the native UI3 that is baked into
Blue Iris. It is free. Simply open up a browser and type in the IP address of your BI computer followed by :80 or whatever port number you use for BI.
What the paid BI app has over UI3 is that if you have two-way talk cameras, you can only use that with the paid app as it is not possible with UI3, and the paid app can do push notifications that UI3 cannot do.
Many though will use the Pushover app ($5) and use that to get push notifications. The benefit to this is you will get images whether you are on your network or not. With the paid BI app, you only get images when away from your network if you port forward or keep a constant VPN connection open.
I use UI3 the most, but I do have the paid app as well.