Though he likes to write letters to thousands of CEOs at once, Larry Fink must flinch these days when one lands on his own doorstep. In recent months the boss of BlackRock, once feted for “democratising” access to investment, has received stinging missives from Republicans and Democrats alike. “Dear Mr Fink,” started one from 19 GOP state attorneys-general on August 4th, accusing BlackRock of selling its customers short by pursuing an…