Daario’s description is ambiguous except for fair-skinned and blue-eyed. Now… to me, here’s a person that fits that description (and he’s not white):
The description in the books is a guide that the show might not adhere to. Anguy’s described as having red hair and freckles, but he’s dark-haired. Tyrion and Jaime have ceased to be blonde haired shits for two seasons now. The show doesn’t stick to descriptions 100%.
Daario’s description in the books really doesn’t matter in the end. What matters is that the casting call specifically asked for any ethnicity except white. I know for a fact that actors of colour did try for the role (one of them I know of has dark brown hair, dark brown eyes).
It’s just a very common occurrence where a production sends out a casting call for an actor of colour and white actors will also reply thinking that they can get a tan and just go for it. It doesn’t work exactly the same way for actors of colour who have to deal with most castings that explicitly state “white only” or imply it; many of them can’t just show up and steal that part. They are often even discouraged from applying to castings specifically geared toward them because they’re so used to being rejected (if they don’t fit a stereotype) or used to so many roles being whitewashed.
Since that actor did win the role of Daario, I was expecting him to be really dynamic on screen and he was just boring. :-/ I don’t know if it’s the dialogue or what, but meh.