Is it blind faith to trust someone else's word?

No. Someone who believes a person because they have long known them to be truthful is following their experience. Faith only becomes blind when it is held on to against all of one's own experience.

Imagine a good friend tells us that there is a fire behind this wall. We cannot see the fire, yet we believe him. That is not blind faith. We have known this person for years, and he has never lied to us. Our own experience of him becomes a reason, and that reason works like a logical conclusion.

This is how every person learns most things. No one can check everything for themselves. We rely on people whose reliability we know. That is why the second verse above tells us to check a report carefully when it comes from someone who is not trustworthy. What matters is not whether we believe someone, but whether we have good reasons to believe them.

The account above shows both sides. The people of Mecca confirmed that they would believe the Holy Prophet Muhammad (saw) even if he warned them of horsemen they could not see for themselves. Their experience told them he was truthful. But when he warned them of God's punishment, many turned away from him. Their blind faith in their idols was stronger than everything they had experienced of him.

This is where real blind faith lies. Someone who trusts a proven witness is not blind. Someone who clings to a belief for which they have no reasons, and pushes aside their own experience to do so, is. Islam invites people to take the first path and to avoid the second.