Are there any dark colored messenger bag at a decent price?

Definitely there is a dark colored messenger bag that you can buy at a decent price. If you are going to put lots of your stuff in just one bag and you like it to be messenger bag then I found this nice Heavyweight Classic Messenger Bag for $15.38 - $25.65 depending on the color you want. There are 4-5 dark colors you can choose from. This bag has large front pocket, 1 side pocket on each side,  3 outside pockets with adjustable shoulder strap. Its size is 15" x 11" X 6".

If you are going to use it for your laptop then here is Solo 17" Laptop Messenger Bag Black for $32.88. It has full length zippered main pocket under flap with large adjustable speed buckle secures front flap. It also has organizer and accessory pockets under front flap with uniquely styled shoulder strap that features a non-slip pad. The back of bag features a ticket pocket and Ride Along Pocket for consolidated travel.
   
Another one is this Greatland® Black Messenger Bag - Black for $15.99. This is a sporty, scholar messenger bag with durable 600-Denier Polyester construction , Polyester lining. It has 1 main compartment, buckle closure. Its exterior features mesh pocket, elastic closure while its interior includes padded laptop compartment, velcro closure. Its dimensions is 17"x5"x12".