it's always possible to get fake negatives for hpt's and blood tests but....blood tests tend to be very accurate.
i would say go to your ob/gyn or gp for an exam and additional blood work to see if there may be some other reason why you're not having your period. pregnancy isn't the only thing that causes menstration to be delayed. stress, sickness, std's, sudden weight gain, sudden weight loss are just a few.
Your probably 95% not pregnant but I would wait another week and take another test to be sure and try not to worry about being pregnant and your cycle may start. Good luck :)