The Gemara on Yevamos page 15B says that you need intent for liquid to make something mekabel tumah. But when would you intentionally put blood on something (blood being one of the liquids which makes something mekavel tumah)?
The intent does not necessarily have to be for the liquid to rest on the food item itself, Rather, whenever the extraction of the liquid from its source (blood from the body) was done with intent, it is considered as thought it is "b'Ratzon," and if it then falls on food, it is Machshir it to become Tamei (see RASHI in Chulin 37a, DH l'Inyan Hechsher).
Therefore, we do find cases of blood being taken with intent, such as in a case of blood-letting, which is considered "b'Ratzon" (Machshirin 6:5).
(Similarly, we find in Chulin 85b that when one draws blood in order to use for removing the "Yeniva" blight, this blood is considered to be Machshir l'Kabel Tum'ah. Similarly, we find in Chulin 33a that the blood that comes out at the time of Shechitah is Machshir l'Kabel Tum'ah.)