I haven't seen many people blogging about getting started with Metal since it was announced in June. Obviously the big game engines are all embracing it but if you want to understand it at a lower level point of view, Ray Wenderlich has written about getting up to speed from first principles.
Mathew Sanders with the second part of an excellent article on interactive view controller transitions. Not only is it a well written and illustrated article, it's got sample projects at every stage of the process. The fact that it's in Swift isn't the point here, it's just a great article on what can be a tricky thing to get right. Also check out Part 1.
If, like me, you learned about Garbage Collection a long time ago and then haven't really looked back at the details since, this article is definitely worth a read. The excellent visualisations really helped me to understand what was going on as well. Naturally, an article about garbage collection isn't directly related to iOS development but consider this something you should read to round out your knowledge of memory management techniques.