It doesn't say it but it shows it. Itachi coughs up blood In his hand when itachi pokes sasukes forhead he falls and drags his hand down sasukes face. The blood slowly went into his left eye then his eye started bleeding
Itachi coughs up blood
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Itachi pokes sasukes head and the blood is on his face
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The blood goes into sasukes eyes and awakens his ms
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If he did supposedly get it during the loss of someone close to him why didn't get it when his clan was killed?