Why is everyone tense as scene eight opens? scene eight and nine

At the end of Scene 7 something broke, and Blanche could feel it. She sees the result immediately in Scene 8, when Mitch doesn't come to her birthday party. His absence is felt deeply by all involved, an elephantine black hole in the room. Blanche knows what has happened but cannot bring herself to acknowledge it – she telephones Mitch and worries that something has happened to him, but she knows she has been stood up. She uses the age-old technique of chattering away to avoid the conversation that is about to happen, whether she likes it or not And when Stanley leaves the room and Blanche begins to question Stella, it is clear that she also knows why she has been stood up.