Black female professionals have seen a steep drop in employment over the last year. They are turning to each other for pep talks and résumé advice.When Nneka Obiekwe started a WhatsApp group chat in September, she was just trying to help her out-of-work friends. A consultant and skilled networker, Ms. Obiekwe, 37, is the kind of person friends turn to when they are looking for a referral or connection to a hiring manager. But since March, she had been receiving pleas every few weeks. By autumn, her network was tapped out. Most of the people reaching out were, like Ms....