What is this birthmark?

When my son was born he was born with a birthmark that starts at his ankle and finishes on his bum cheek.. I was told it's Mongolian blue spot.. It's a reddish purple most of time a reddish colour and I've seen pictures of Mongolian blue spot an they are a bluish colour.. His birthmarks gets more purple when he gets warm or gets a warm bath.. Will it go away?