You may experience a blue screen when connected to the external device (such as DVD, DVDR, Set top box,etc) with a video format that does not comply to the television supporting video formats.
This TV support only the video format of 50Hz and 60Hz refresh rates. The source device (DVD, Set top box, etc..) should actually read the TV capability, and only reflect to the user the formats (including refresh rates) that is supported by the TV.
You need to change the correct video format on your external device (such as DVD, DVDR, Set Top box,etc) that the TV support. Please refer to the external device user manual on how to change the video format.