Keep in mind that many checks are being cashed via scanner or photo. These can be home based, business based or ATM based systems. The key requirement is that the software has to be able to distinguish the "written" parts from the background parts.
If the image doesn't have enough contrast for the edge detection to work, the check can't be easily processed. In that case a human looks at the image and decodes the image and processes the transaction. The image is not in color.
Many businesses scan the check and hand the original back to you after having the Point of Sale system process the image.
Post 2001 the checks in the united states are no longer moved through the banking system, only their images. With the roll out of these image based systems, in the future almost no physical checks will be seen by banks. Therefore the actual ink color is not important, only the result.