So I'm starting to think that Haki is only coloured black for the sake of the reader. Here is Rayleigh's explanation of Haki:
As we can see, the Haki that is capable of touching a logia is actually invisible, and Rayleigh describes it as an "invisible armour". So there are 2 possibilities.
1. The black coating of Haki is only done so the reader can understand when Haki is being used.
2. Perhaps you can only see the black coating if you have been trained to use Haki or are familiar with it.
I'm thinking option 1 honestly. You were correct in your assessment of the fight with Caesar. I believe in 680 Franky even points out that Luffy used Haki to grab him, but he had to question whether or not Luffy was even using it.
There is contradiction in this exchange:
I don't have a good reason for Luffy not using Haki in the first panel. But if you go by the idea that Haki is not a physical coating, then it's possible that he was using it there, or he truly thought he could hit Caesar without Haki. As I did say, you can only use logia powers if you will your body to do so. Perhaps Luffy thought that he was so much faster than Caesar that he could attack him before transforming?
Think about Luffy vs Crocodile. Crocodile had awakened his fruit and was said to be a master of it in combat. It's reasonable that he'd be far better than a scientist like Caesar at transforming his body into sand to avoid being hit. Caesar isn't acclimated to high level combat, so he may have been dodging base Luffy's attacks out of fear that he would slip up.