My Coworker's Cat Swallowed String About 1 Week Ago (how

My coworker's cat swallowed string about 1 week ago (how much, she's not sure, possibly a LOT). He appeared fine until the night before last when began vomiting throughout the night (4x) and would not eat or even come out from under the bed the next morning. She took him to the vet where they kept him for observation but did not do X-rays, only a blood test! His temperature is NORMAL! WHAT are they 'waiting for' and what will a blood test show IF it IS an intestinal blockage??