When Dorothy first talks to Mr. Gordon on the telephone, she tells him to meet her at her house, but doesn't give the address. If this is the first time they've spoken, Mr. Gordon wouldn't know where to meet her.
The whole joke of the episode turns on Rose not understanding the meaning of the phrase "sleep with someone". But Rose herself has used the phrase correctly numerous times in previous seasons, both for herself (when she was convinced she killed men by sleeping with them) and for others ("Ted was your date, Blanche! Dorothy, you must have slept with someone else!")
Rose asks Blanche to turn on the heat for her if she isn't mad anymore. She says she was chilly. It is rare for anyone in Miami to need the heat, especially how they are dressed for the episode. Obviously no cold front for Miami at the time.
No one who knows Blanche would believe that she's gay.