。
She decided on blue clothes.
Literal
She [topic-は] blue clothes [on-に] decided.
~に決めた means 'decided on' — the に marks the chosen option from among alternatives. 服 (ふく) covers clothing in general. The sentence implies a shopping context where multiple options were available and blue was the final choice. 決める is one of the most useful decision verbs: ~に決める ('decide on X'), ~と決める ('decide that X'), 決めた ('it's decided').