When issuing an identity in the Desktop Wallet, the user can choose to switch to a phone by establishing a secure link via a QR code. Scanning the QR code then opens the default browser of the phone. In case the screen stays black and does not allow to take pictures of the id documents, the default browser must be allowed to access the camera.
To check or change your default browser, go to Settings, select a browser (e.g Safari, Chrome, or other browser), check or change setting for Default Browser App.
To allow the browser to use the camera, go to Settings, select the default browser, enable Camera.
Some more background information:
From iOS version 14, it is possible to change the default browser away from Safari to any other browser, e.g. Chrome. For other browsers than Safari, there is no option in the settings to make the browser ask for permission to access the camera. The initial setting for those browsers appear to be camera access disabled. In that case, when attempting to take a picture of the id document, the user is presented with a black screen where the photo button does not work (see above).