I want to make sure that we are taking about text messages in the Apple Messages app on your iPhone. You are right, that if you Touch and Hold a text then the rest of the screen within the text message window is blurred except the test that was chosen.
Let's double check that the Reduce Transparency Mode is not set to ON.
Open Settings ➔ Accessibility ➔ Display & Text Size ➔ Reduce Transparency: Ensure that this is set to OFF.
If the above is set correctly, then Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:
- Press and quickly release Volume UP button
- Press and quickly release Volume DOWN button
- Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF).
Axel F.