Black.
Also, do some googling on how blue dials pair with different leather straps. For me, I don't like the look of blue with black leather strap. Blue with brown - dark, reddish, or even "toffee" brown, fine, but throw a black strap against a blue dial, and it doesn't work for me.
Black can take dark browns, and even some reddish browns, but not light brown or "toffee" at all. So it goes both ways.
But black wears with just about anything. Blue not necessarily. Don't get me wrong, I love blue, but not in a dressy/sporty hybrid (like an OP-style watch).
However, I think white, off-white, or cream dials are superior to them both. They "pop" easier, but not in a garish way, and they truly go with virtually every other color you could wear without clashing. To me, black can clash with other colors like navy blue. I grab my white dial dressy watch over my black dial 2 to 1.