What is the most sensitive phase for detecting active bleeding?
Portal venous or delayed phase - it simply allows more time for contrast to accumulate, making it easier to see.
Should positive or negative oral contrast be used when investigating for bleeding?
Neither - positive oral contrast will obscure bleeding and negative oral contrast (i.e. water) can dilute the extravasation. Both of these can lead to false negatives.
Red arrow indicates the site of contrast pooling in the antrum of the stomach. This on seen on this portal venous phase image.