I don't know much about Amazons, but I have seen several at the rescue I volunteer at with that exact black edge/outline to them that are perfectly healthy...
What your friends were speaking about with black markings on the feathers being a negative issue usually appears as black "barring" or bars across the feathers themselves. These can result from stress, poor diet, boredom, etc.
If you are not able to spend much time with your bird, and he's inside of his cage most of the time, please make sure that he has lots and lots and lots of different types of toys, at least 6-10 different types of toys at any given time, and make sure that you rotate them through with new toys each and every month. In addition to the many different toys, he also should have some different "Foraging Activities" inside of his cage, such as cardboard boxes filled with crumpled-up paper, pieces of cardboard, newspaper, etc. and treats/nuts hidden all throughout the box. Also, you may want to start leaving the TV on for him during the day in a spot he can see it, maybe to the National Geographic Channel, Discovery Channel, Animal Planet, etc., or if he likes music then leave the stereo on for him.
Amazons are extremely intelligent parrots and they can become bored inside of their cages even when they person is home all of the time. So if your guy is inside of his cage most of the day, then you need to keep him really busy, occupied, and most of all, he needs to be intrigued and made to think. Think of it like you need to give him "jobs" to do. He needs to be kept from becoming bored as well, which is what the lots of toys inside his cage, and making sure that each one is a different type of toy is about...one toy that is for "shredding", another toy that is wooden and for "chewing" and "breaking apart", another toy that is for "thinking" and "using his beak and feet" like a toy with beads or other parts that he can move, another toy that he can "swing on" or "hang from", such as a swing or a rope-boing, etc. I know that bird toys are expensive, especially the ones of larger birds, but they are much less expensive than a Certified Avian Vet is, lol...
And making him the "Foraging Boxes" is not only cheap, but will keep him occupied for hours and hours. My Senegal absolutely loves the Foraging-Boxes I make, he totally ignores me when he has one, he doesn't care that I'm even in the room, lol. When I make him a new Foraging-Box and he first sees it, his eyes light-up like a Christmas Tree, they pin like crazy, and then the yelling at me to give it to him starts. So i put them in the bottom of his cage and he immediately goes to work.
Usually what I do is find as large a cardboard box that will fit inside the bottom of his cage, and then I find as much paper, newspaper, pieces of cardboard, packing paper, anything that is safe for him, that is not too heavy for him to move, and that can be shoved inside the box. I just start crumpling-up the paper into balls and start throwing it all inside the box, and intermittently I hide nuts inside the shells throughout the box. Then once I fill the inside of the box to the top, then i close-up the box, so that he has to first figure out how to open-up or get into the box before he can even start foraging. And he will work on that box all day long, sometimes it takes him more than one day to get through to the bottom. And it really does become his "job", he works feverishly at it, stopping only to eat a nut that he find inside the box, to get a drink, and sometimes he will actually take a small break to go play with one of his toys, like he wants to chew on a hard, wooden toy and shred it apart for a while, then he goes back to the box...It keeps him happy, healthy, entertained, and most importantly stimulated...
The last thing you want is for your guy to develop any anxiety, stress, neurotic issues, etc. because he's bored, lonely, etc., which can happen when suddenly their person is not around much at all. I know that it's sometimes unpreventable with work that this happens, hopefully it's only temporary, and hopefully you are at least letting him out of his cage for like an hour before work every morning and then a few hours at night after you get home, try to do the best you can to spend any spare time you have with him...And again, please, if you haven't already, get him a ton of new, different toys, along with different Foraging-activities or "jobs" for him to do...you want him to keep his beak busy working and playing instead of starting to pick at himself too, which is another issue that often starts this way...
And be sure to ask any questions that you may have here, there are some fantastic, knowledgeable, and greatly experienced Amazon owners here that can help you with pretty much anything.