Dark prismarine is miscolored on maps

The Bug:

Dark prismarine is miscolored on a map. It displays the same color as prismarine bricks do.

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Steps on how to reproduce:

1. Put some prismarine bricks down on the ground, and then put some dark prismarine next to the prismarine bricks.

2. Make a new map.

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The attached images show the ground completely covered in prismarine bricks and a structure with a dark prismarine roof.

I'd expect the structure's roof to have a dark color on maps to distinguish it from the prismarine bricks on the ground layer.

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