。
She had long blonde hair.
Literal
She [topic-は] long blonde [possessive-の] hair [object-を] was-doing.
ブロンド is a loanword from French/English for blonde hair. 長いブロンドの髪をしていた uses the ~をしている pattern for describing physical features, here in the past tense. The mention of blonde hair implies a non-Japanese person or someone with dyed hair — either way, it stands out as a notable physical characteristic in a Japanese context.