The 27-year-old Gossip Girl actress was due to give birth early this year but had the baby early at their home in Bedford, New York before the New Year.
The celebrity website Page Six said both mum and baby were 'healthy' and 'doing well'.
Blake's letter read:
"Preserve at its core is about family. Family is the single word our founder, Blake built this home on.
"We have something for men, for women, for the old, but we realized, we don't have anything for the new. With family on our mind, we looked to the origin of it all-to the women in our lives who are right there, at that special moment; at the creation of family."
The actress, who married the Ryan in front of just 35 guests in 2012, previously told Vogue about what an amazing father she thought he would be.She said last year: "He's going to be a great father and patriarch - he's so meant to be all those things."
Meanwhile Safe House star Ryan insisted he would have no trouble with daddy duties.
He told PEOPLE: "I don't have a weak stomach. I'm good with diapers and diarrhoea and all that kind of stuff because I have lots of nieces and nephews, so I've done it all before."
He added: "I'm just excited about having a buddy. We don't know if it's a boy or a girl yet, but whatever it is, you have this tether to reality that never goes away."