[Crex]

Studying the pics, it looks like you are trying to jump to the 400 grit too soon. There are a lot of deep scratches in the surface that the 220 grit should have removed completely. Running at Griz speed you may need to add in a 320 grit step to get rid of the scratches if they are from the 220 belts. If they are from a coarser belt, say 120, then you are not through with the 220 before going to 400.
Think about it - if you didn't get them out with the 220.....400 hasn't got a chance.

Belts are either "clogged" or dulled to the point that the steel is getting too hot and oxidizing to a discoloration level. Try using a sanding block and fresh paper, it will take a little elbow grease but should remove the discoloration. Griz's run real fast and it doesn't take much pressure/run to dull a belt.
Are you using a magnet to hold the blade to the belt? If so you may not be feeling the heat build up before it's too late. Don't wait until it burns your fingers to dunk/cool it.