Hi there! I understand you're watching BritBox on your iPhone running iOS 18.6, and sometimes the screen goes black with a blue square turning on. Let's get to the bottom of this together.

To start, could you tell me a bit more about this issue? For example:
- Does it happen with any specific show or all content?
- How often does the screen turn black—every time you watch, or just occasionally?
- Have you noticed if it happens more when you're on Wi-Fi or cellular data?
- Also, do other apps on your phone stream video without issues?

Once I have a bit more info, I can guide you through some steps tailored to your iPhone to fix this. Meanwhile, here are some initial steps you can try:

1. Restart the BritBox app: Close it completely by swiping it away in the app switcher, then open it again.
2. Restart your iPhone: Sometimes a quick reboot helps resolve temporary glitches.
3. Check for app updates: Go to the App Store, search for BritBox, and see if an update is available. Updating can fix bugs causing display issues.
4. Check internet connection: Make sure your connection is stable. Try switching between Wi-Fi and cellular to test.
5. Reinstall the app: If the problem continues, delete the BritBox app from your iPhone, then reinstall it fresh from the App Store.

If you've tried these steps already, let me know so I can suggest more advanced troubleshooting. Also, if you're willing, we can check some iPhone settings related to video playback or software conflicts.

Looking forward to your reply!