You add 9 volumes of water (or buffer) to 1 volume of blood.
The units don’t matter as long as they are the same.
You could add 9 mL of buffer to 1 mL of blood or 9 L of buffer to 1 L of blood.
You still have 1 volume of blood diluted to a total of 10 volumes.
This is a 1:10 dilution.