Hi Karen, your puppy looks very sweet. Two general points re coat colour:
Hand stripping is definitely the best way to maintain the colour of a schnauzer coat. Even pepper and salt minis can lose the pepper and salt colouring and end up with a pale silver or grey dog if they are not hand stripped. This is because clipping or shaving the hair removes the harsh top coat and exposes the undercoat which is often a paler solid colour and softer hair.
Any mating of two different coloured schnauzers is unpredictable as regards how the puppy will develop. Again, even e.g. a mating between two P&S schnauzers can produce puppies of a very light or very dark colouring as they mature but with two completely different colours the mature dog’s colouring is never certain. True Black or Black and Silver mini schnauzers are only bred from lines on both sides with the same colour.
However, as regards your little one, he is still very young so the harsh adult coat will not be evident yet, especially as he has been clipped. As his hair grows longer, if you can start stripping out the soft grey puppy coat you may find that he does have a good black coat coming through so don’t despair or think he will remain grey. From the second photo he does look to have black coat where it has not been clipped. The grey coat on his head and body is just the undercoat and hand stripping should bring back his black top coat
