When should i get a blood test done to determine pregnancy?

AF is 4 days late. My cycle has never went this long. It's always on time. I keep getting bfn. Should I wait longer before getting a blood test? I don't have any major pregger symptoms just a little bit of nausea and insomnia.  I'm not getting my hopes up because I feel like AF will show. Yesterday afternoon I wiped down there and had a bit of brown stuff -thought it was the beginning of AF and nothing started its now early morning the next day.