I am absolutely not a snake expert. I know the black racers, because I have one run that runs back and forth along the wall in my back yard. With my very limited knowledge, I would say that, from what I can see from your picture, the snake's head does not appear to be that of a venomous snake. A good website is
Snakes of Florida | Florida Backyard Snakes. It shows common venomous and non-venomous varieties.
With what others have said, I will tell you that I've had several friends that have let their cats roam and the cats eventually do not come home.