I agree with Rosa anything is possible, but blood is pretty accurate. I was due for my period the day after my wedding. I was SO stressed from the event that it never came. I was for sure I was pregnant even though I was on the pill. That was the only time I have ever missed it and NOT been pregnant.
The blood test is pretty accurate!! Are you under any stress?? That can delay a period or an infection can do it as well! I would talk to a doc about him giving you something for you to come on your period! But to tell you the truth i have seen it happen before...one girl on here had neg blood test and neg urine test...she had an ultrasound done that showed she was indeed pregnant so i guess its possible!! Good luck