BlackRock's $1.625 billion apartment purchase in Southern California has alarmed thousands of local renters, prompting new concerns about life under a landlord backed by one of the world's largest investment firms. For renters already struggling with high housing costs, the biggest fear is straightforward: higher monthly bills, as the New York Post reported. Here's what to know Announced Aug. 5, the transaction covers 11 complexes with 3,620 apa…