I think blood tests are like the first response pregnancy tests which you can take like 4 days before your period is due, but now, in britan anyway, doctors just send urine off to the hospital for analysis to see if your pregnant which would need to be the day your period is due or later i assume.
If your period isn’t due yet or is due soon i’d just wait until then and a couple of days after. the chances of getting pregnant from having sex 1ce are very very slim… takes most people 6 months or more of regular intercourse to get pregnant.
Lolaxx