Learn English – What does ‘outsize character’ mean

The most reasonable parsing of that particular sentence would be that "it was the sole dialogue available in Europe for almost 1,000 years" could plausibly be the only reason for "The influence of the Titnaeus among early philosophical thinkers" being "pervasive", without explicitly excluding any other reason.

This is in reference to a 'blue movie', a euphemistic term for a pornographic film.

It has since been toned down somewhat and phrases like a bit of blue can be used to say that something is 'adult' in nature. Often this includes dealing with sexual material but doesn't necessarily mean visually pornographic; a comedian who tells jokes with a sexual theme could be called 'a bit blue'.

going a bit blue then would mean that the programme/character in question is normally 'clean' but has begun to include more adult themes.

It would appear that the term has been used thus since the early 1800s, originating in Scotland, though a clear connection between the colour and the connotation has not been settled on.

N.B.: Bolton comedian Peter Kay is famous in England and well known for his family friendly stand up material. In his shows he will occasionally tell a joke with some sort of subtle sexual reference or other 'adult' theme and then follow the joke by saying 'a bit of blue for the dads there'.