"A pregnancy blood test is considered to be about 98% accurate. When the test is negative but pregnancy is still suspected, the test should be repeated in 1 week." Taken from (http://www.nlm.nih.gov/medlineplus/ency/article/003509.htm#Special%20considerations)
According to womenshealth.gov, the Federal Government Source for Women's Health Information: "Blood tests can tell if you are pregnant about 6 to 8 days after you ovulate"  About urine tests they say: "In a 2004 study, researchers tested the accuracy of 18 HPTs sold in retail stores. They found that only one brand consistently detected the low levels of hCG usually present on the first day of the missed period. This was the First Response, Early Result Pregnancy Test. The other tests missed up to 85% of pregnancies on the first day of the missed period. Most tests accurately confirmed pregnancies one week after the missed period." (http://www.4woman.gov/faq/pregtest.htm#b) So a blood test is more reliable than most home (urine) pregnancy tests in the days right after ovulation occurs. Later on, they seem to have a similar accuracy rate.
Also check out the nurse forum questions "am I or am I not" and "trying for baby number 2" for further discussion of this. I would retest in a few days. Check the website above for more comparison of the different tests. Let us know what happens! :-)