Differences between the novels and TV series:
Blair's mother is a wealthy hostess in the book, she is a designer in the TV series.
In the book Blair has blue eyes while in the TV series, Blair has brown eyes.
Blair has a cat named Kitty Minky in the book. It is note worthy, though, that in the TV series, Roman, her father's lover, named their cat Cat, in reference to 'Blair's favorite movie'
Blair's mom dresses very strangely in the books, whereas in the TV series, she is a fashionable woman.
In the books, Blair's father's male lover's name is Giles whereas in the TV series his name is Roman.
Blair's favorite store in the books is Barney's New York, whereas in the TV series she wears various designers.
In the book Eleanor Waldorf is dating Cyrus Rose in the TV series Cyrus Rose isn't mentioned.
In the book series, Blair has a younger brother named Tyler, whereas, in the TV series, she is an only child.
In The TV series, Blair's mother remains single, whereas in the book series, she is married to a man called Cyrus Rose, also inheriting a stepson, around the same age as Blair.
(found in link