Trowel on bitumen is a dampproofing material and will not waterproof the wall. Also, bituminous materials cannot be left exposed to sunlight.

The choice of waterproofing materials depends on the surface you are waterproofing, how it will drain, and the cladding that will protect the waterproofing. You need to describe the conditions better.

The bitumen is designed primarily for patching roofs, not walls.

First, find exactly where the water is coming in. The roof would be the more likely source than the wall. Then use the appropriate materials/techniques.

Black Jack makes bitumen-based fiber-reinforced roofing cements, mastics, paints and foundation coatings. You don't say which one you are considering but I suspect there is a more serviceable material.

Thanks for the replies. The wall is about 3 feet high by 20 feet long, and just inside the wall is the gutter. There are a couple of cracks in the cement, otherwise it's sturdy.

Would the best solution be to use the Black Jack as an adhesive, and to stick roofing felt to it? I also saw another product which can be applied with a (paint)roller. It's for sealing a driveway and is also supposed to be waterproof.
The leak appears in a bedroom just under that wall, and a neighbour's house also has some black product on his piece of wall. I think it's not a very good design, having 3 feet of wall going higher than the gutter...anyway...

You need to tell us the relationship of the wall to the roof and what the gutter is for etc. A photo would be a good start.

I dont have my camera here - might borrow one. But it's an old (maybe +70 years) house. Looking at the house front, the wall goes about 3 feet higher than the gutter, which is inside the wall. The actual gutter seems ok, and is a few feet away from a pretty good downpipe. The wall itself looks ok, but with a few fine cracks in the rendering (cement). Would that driveway sealant be any good? presumably if cars can drive over it, it must be strong? I appreciate the help.