That's still possible because blood tests can detect hcg from 1 to tens of thousands or more ( obviously not that high that early just saying)
where as urine would still show no reaction to hcg tets until the test threshhold is met - lowest I have see is 10. a + hcg in blood of 5-7 say may not show on a miu test, and most tests at stores are 25miu. So technically you would be preg and it shows in blood, but not on pee test.
Don't bother yourself too much about it if the blood is pos and pee stick neg. Just give it 2-3 more days and the urine should catch up as long as hcg keeps rising.
GL!