Plan b pill and blood work

Not sure, most of the times blood tests don't test for everything and are looking for specific things so I think it is unlikely, but more importantly I think your concern is in the wrong place. You should be asking if the plan b pill will interfere with the test he is trying to take as some medication does that.

In your shoes I wouldn't worry about the Dr knowing that and would actually disclose it to the Dr to make sure the blood tests are accurate for what you are being treated for. The Dr doesn't care about your plan b pill use and you shouldn't care about your Dr knowing.