。
She ended up with a blister on her left finger.
Literal
She [topic-は] left-hand [genitive-の] finger [on-に] blister [subject-が] formed [completion-てしまった].
まめ ('blister') is one of those Japanese words written most naturally in kana — the kanji 豆 ('bean') feels too literal, since the word also means a literal bean. ~ができる ('to form, appear') describes things that come into being on or in something: 痣ができる (a bruise forms), シミができる (a stain appears), まめができる (a blister forms). The ~てしまう closes with the regret/completion shade — 'ended up,' implying the result was unwelcome. The に on 指 marks where the blister formed, an existential location.