Idaira Chavez is just the type of car buyer that BlackRock’s new social-impact fund might have had in mind when it bought a stake in Tricolor Auto Group. Tricolor gave Chavez, 21, a loan to buy a 2012 Kia Optima off one of its lots earlier this year. She needed the car to get to her job at a Dallas pet store after she moved to a nearby suburb. ... But Chavez says being a Tricolor customer hasn’t made her life any better: A few months after buying her car from the company in January, the vehicle stopped working and...