The blood clots and seals the hole in the dura which stops fluid from leaking out. The procedure is used to relieve severe headaches caused when an epidural, spinal needle, or diagnostic lumbar puncture punctures a hole in the dura.
It is has also been suggested that immediate relief of headache after a blood patch is due to pressure against compression and squeezing effect against the dura which relieves the low pressure in the head.
Usually you are monitored in for around one hour and discharged shortly after. Often, relief is immediate, but it can take a day or two to return to normal.
It is important for a few days to try to avoid coughing, sneezing, laughing and straining anything which could dislodge the clot. Try to rest for a couple of days and give the blood patch time to let the hole in the dura heal.
You may get pain in your back, radiating down your legs for a couple of days after because the blood is an irritant. There are other risks involved including the same risks as having an epidural plus a small chance of developing meningitis because blood can contain organisms and because of this a blood patch is not advised in the case of a fever. However, if a csf leak is not fixed, there are also risks associated with that including subdural hematoma. Please read Epidural Headache With A Late Onset for more details.
There is a high chance of failure of epidural blood patch if it is done within 24 hours of onset of headache, success is increased markedly after 24 hours.