If you sign a route you should know where your signals are.
True, but infrastructure is supposed to be designed to be fool proof as it is accepted that humans make errors. This is why AWS/TPWS was brought about, for example.
I imagine that if a blank signal is found to be the cause of this, most members of the public would expect RAIB to change a regulation to minimise the risk of a blank signal causing an accident again in the future, especially given most members of the public would likely sympathise with the driver for not noticing a blank signal at night/within a tunnel.
I’m no engineer but solutions I can think of off the top of my head:
• Put big, reflective outlines around signals
• Have a battery back up on signals which powers a red light if it detects that the electricity is cut (similar to how most modern digital alarm clocks come with battery back up power which only kicks in in the event of a power cut).