There is no guarantee that a deal will materialise and talks between the parties could still fall apart, the people added.
Ionity, whose owners include Volkswagen, Daimler, BMW and Ford, in March said it was examining ways to expand its network of fast-chargers across Europe.
Sources told Reuters in June that Royal Dutch Shell and Renault were among those interested in taking a 20-25% stake valued at 400-500 million euros.
BlackRock declined to comment on the investment, which was first reported by manager magazin.
A spokesperson for Ionity declined to comment, saying that the company was constantly discussing additional investment possibilities with its shareholders, which also include Hyundai Motor, and that additional investors were welcome.
(Reporting by Christoph Steitz, Editing by Nick Zieminski)