It depends on which blue and which brown you use. There are a wide variety. For example if you mix ulra marine blue and burnt umber you will get black. Adding blue to a reddish brown will make the brown darker, but also more violet. Using a yellowish brown and blue will give you more of a greenish brown.
You have to experiment with the colors on your pallet to see exactly which one you get.
The related link below will help you understand more about mixing colors to get the ones you want.
teal or a muddy brown.
Green I think?
teal
Hm wanna meet up
Green
Black
Grey?
Green
W
brown
turquoise
brown
Brown
Grey
brown
Brown Brown
A Civil War re-enactment.More seriously, one such blue-gray blend is called periwinkle.Lavender
cheese
purple or brown
When black and blue are mixed together they will make very dark blue. This is called a shade of blue. If you use mostly black you may not notice that there is blue in the color.
Brown