She looked down but didn’t realise the blood on her knee.
She looked down but didn’t notice the blood on her knee.
Use notice + person/thing when you see, hear or feel something: ‘I noticed some blood on my knee.’ ‘Did you notice her new haircut?’
Use realise + that-clause when you understand something or become aware of a situation: ‘She didn’t realise that she was bleeding.’ ‘I realised that I had made a mistake.’
We do not normally use realise + person/thing when talking about something that we see or hear. In this context, use notice.