...carrying a yellow shield with a blue sigil. What does the blue sigil represent?
Sorry, I'm going to be an irritating pedant now... And go off at a tangent...

'Sigil' is a term coined by G.R.R.Martin in his Game of Thrones books. (Presumably 'badge' was a bit too ordinary-sounding).
'Sigil' comes from the latin word for seal or signature, and was later applied to occult or magical runes/characters - it is not a correct word for a heraldic crest, blazon, charge, whatever...
And since it's a premeditated misuse of the word by the author (much as I love the GoT books) I seriously hope it doesn't start to become used to talk about real medieval heraldry.
Like most fantasy authors, George deliberately mucks about with words and ideas to make his fantasy setting seem a little bit different and otherworldly to our own. Which is why, presumably, his knights are all styled 'Ser' rather than the historical 'Sir'. It's just a conceit.
Sorry to be a nit-picker. Really
