if my dog had internal injuries, would he have died by now?

[morningstar]

my bf took my puppy dog to the park on monday, almost 5 days ago. he let it off the lead, then some bigger dogs scared him and he ran home across a busy road. my bf didnt see him get hit and wasnt at the scene on time but heared the car stop suddenly, and beep, the dog to yelp a few times as it ran away. he was not sure if it was hit or not. a man on the street said he thought it was hit (but he is a weirdo, a possibly on drugs, talks to himself and walks around my neighbourhood) my dog ran home and my bf met him at the door and he was wagging his tail being normal. the last few days ive thought nothing of it until the last time he pooed it was black. which may indicate blood. although i have seen that colour in the past as a very young puppy. could i just be worrying? or could he have internal injuries that may not have killed him yet after 5 days? if he had internal injuries wouldnt he be dead by now? he seems fine. and yes i will call the vet tomorrow but its closed now i cant sleep