。
She was wearing a dark blue scarf.
Literal
She [topic-は] [dark-濃い] blue scarf [object-を] was-doing.
Note the verb する rather than 着る — for accessories like scarves, ties, watches, and jewelry, Japanese uses する ('do, wear'). スカーフ is a katakana loanword from English. 濃い青 ('dark blue') is the i-adjective 濃い modifying the color noun.