[Catfish]

In the early war, yes. But it needed a certain nerve to do it, so not all U-boat commanders did.
They also went in from the side, fired, and then ran with the convoy while reloading. In case of a corvette they would run around merchants and finally outrun it. With the later real destroyers or the DDE destroyer escorts it became difficult though.
At night no one would see the sub and ASDIC was useless against surfaced boats. No radar in the early war, very few escorts, sometimes none, or only corvettes that could be outrun at the surface with the faster U-boats.
Once in the convoy the lookouts spotted merchants and escorts earlier than vice versa, the boats could glide into the shadow of a merchant and out of it, it is reported that corvettes came very close or crossed the immediate backwash without spotting the boat.

With more escorts, and fast destroyers used as escorts, this tactic became obsolete. During opeartion drumbeat it was again possible for a short time, but this was never war-decisive.

After all it was more and fsater escorts, radar, but as a number one threat the aircraft, that made attacks more and more difficult.

I have not read Clay Blair's books, but almost anything else - not sure whether there is much more information in his book? Whatever it was not quite as easy to kill the U-boats as it is portrayed in the propaganda films of the time, or our time.