This looks like it may be a bug. The documentation shows this example:

But the result is not what we expect when executed (v11.3):

Without the ImageAdjust it looks like this:

It is black because it zeros out the values in the dimensions it collapses, for example:

In[1]:= pool = PoolingLayer[{2}]
In[6]:= pool@{{1, 1, 1}, {1, 1, 1}, {1, 1, 1}}
Out[6]= {{1., 1.}, {1., 1.}, {1., 1.}}

In[7]:= pool[{{1, 1, 1}, {1, 1, 1}, {1, 1, 1}}, 
 NetPortGradient["Input"]]
Out[7]= {{1., 1., 0.}, {1., 1., 0.}, {1., 1., 0.}}

So the black strips correspond to the 9 dropped columns and rows from the 10x10 pooling.

However, it looks like there may be a bug in the documentation, which shows this example:

But the result is not what we expect when executed (v11.3):

Without the ImageAdjust it looks like this: