Thanks for choosing doctorspring.com
I understand your concern.
A negative blood test essentially rules out pregnancy, it is quite reliable.
Please get a pelvic transvaginal ultrasound scan done too - it would rule out any pathology in the uterus or ovaries.
Stress, exercise, travelling, late nights, weight changes, hormonal medications, alcohol, smoking - lots of factors can result in menstrual delay.
You should also get hormonal levels - FSH, LH, TSH, Prolactin, OGTT, DHEAS, Free testosterone, to rule out factors like hypothyroidism and polycystic ovarian disease.
All the best
Please feel free to discuss further