The total number of girls in the choir is 80+ 10 = <<80+10=90>>90 girls.
The total number of blondes in the choir after 10 more are added is 30+10 = <<30+10=40>>40 blonde-haired girls.
From the total number, the number of black-haired girls is 90-40 = <<90-40=50>>50 black-haired-girls
#### 50