You can take your blood pressure after having a bath but it's recommended to wait at least an hour after you take the bath before taking your blood pressure.
Warm and hot baths can lower your blood pressure so if you take your blood pressure immediately after taking the bath then your blood pressure may read too low.
If you wait at least 60 minutes after taking your bath before taking your blood pressure you'll get a more accurate reading.
If you suffer from high blood pressure then soaking in a warm bath can help you lower your blood pressure at least for awhile.
Walking, bicycling etc is also good for high blood pressure and is good for your health as well.