Do you know how many DPO (days past ovulation) you are? There is a massive range of levels that are OK so even a couple days could make a big difference between low and pretty good. If you had a super faint HPT that would indicate a low level of HCG which the blood test has confirmed, but if it is still early days it could just be that it hasn't got high just yet. Have you done another HPT test since the one the other day? Might be time for repeat test (HPT and blood) to see what change there has been.
Good luck, I hope this is the one for you!