'Tis a blot on his escutcheon.

The blot represents a fault or disgrace that diminishes someone’s good name. The heraldic shield makes that loss of standing visible as a mark of shame.

The saying treats the person’s reputation as something that can be visibly blemished. It expresses judgment about the damage, not merely the fact that something went wrong.

Use it to describe an act or failing viewed as a stain on someone’s reputation. Its heraldic imagery and shortened opening give it a formal, old-fashioned tone, so it suits deliberate or literary phrasing better than casual conversation.