You can test with the free version first it only let's you setup one camera.
See example on setup from one of my cameras. Once you setup one camera you just need to copy an existing camera give it a different description and change the camera Stream URL to point to the BI new camera short name.
I point to my Blue Iris IP and port not directly to the cameras. I couldn't get many of my cameras working in IP Camera Viewer but pointing to BI they all work.
You can even point to a camera group name and display the group of cameras.
You can also set it up so it is the Roku screensaver if you want to display your cameras.
Or you can find your Roku IP address in the Roku Settings/Network menu and, in Blue Iris, add it to the Limit access by IP address list, starting with a carat (i.e. ^192.168.10.111, where 192.168.10.111 is the IP address of your Roku).