The Rhnull blood type, known as golden blood, is a real thing. Although not quite as rare as in the show, where it's being portrayed as 7 people worldwide, there are an estimated 45 worldwide out of 7 billion people - so still incredibly rare.
The poster of Raymond Reddington in the post office has his birthday listed as February 7, 1960 in Massachusetts. This is the birthdate and place for James Spader.
Red says he was gored by a wild boar in The Forest of Dean (which is in England near the border with Wales). Although boar had been extinct in the UK for about 300 years, in the 1990s some escaped from a farm. In 2004, 60 farm-raised boar were illegally dumped in The Forest of Dean, and now the populations have mixed and are thriving.