It’s not a Senate ‘rule’. It’s a custom in the Judiciary Committee, and its implementation has varied over time.
McConnell isn’t a strong enough leader to insult Grassley by removing Grassley’s decision as to how to apply the blue slip custom. Grassley is far friendlier to the Trump agenda than McConnell. Any allowance or disallowance of the blue slip custom is far better off in Grassley’s hands than in McConnell’s.
Many thanks for the explanations. I did read the Huffpo, and the Weekly Standard, and it was still not entirely clear. The Huffpo pile was of course just a steaming pile...