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:

[![enter image description here][1]][1]

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

[![enter image description here][2]][2]

Without the `ImageAdjust` it looks like this:

[![enter image description here][3]][3]


  [1]: https://i.sstatic.net/Twj6s.png
  [2]: https://i.sstatic.net/IjklK.png
  [3]: https://i.sstatic.net/7qH0o.png