Thought I would mention, that grey blocks do have their use in Midway...

When you pick up the power pellet before a loop the bonus you get from picking up the blocks is based on how full your grid is.

The grid clears when one of two conditions are met...

1. you have gone a short amount of time without picking up any blocks

2. you pick up a block in a column that is already filled.

Grey blocks are actually helpful in preventing both of these occurrences, possibly long enough to hit that power pellet at the best time.

Picking up a grey "resets" the timer, buying you more time before your grid clears.

It also removes a block from a column allowing you to pick up another block in that column, thus, buying you even more time to get to the pellet.