Are blood tests always taken before a chemotherapy session?

It is usual to have blood tests before chemotherapy but not always on the same day. Different chemotherapies have different risks and so different tests need to be done at different times. It is very unlikely that the chemotherapy given on that day contributed to your father's stroke. The effects of the chemotherapy usually take some time to kick in. So while the long term effects of the chemotherapy might have contributed to the stroke, the chemotherapy that day likely did not. Because of that it is unlikely that the stroke could have been prevented as it could have easily happened had your father been sitting at home.