In "My Last Duchess," the "blush" or "spot of joy" is the Duchess blushing. The Duke indicates that this blush is not because the Duchess is embarrassed or shy. Rather, he claims it is a flirtation. Therefore, the blushing "spot of joy" is a metaphor for flirtatious behavior.
Sir, 'twas not
Her husband's presence only, called that spot
Of joy into the Duchess' cheek; (13-15)
The painting itself is, "by design," the Duke's conception of his late wife. In the poem, he complains that she did not reserve her smiles just for him. He is jealous that she would give the same kindness to other men.
Oh sir, she smiled, no doubt,
Whene'er I passed her; but who passed without
Much the same smile? (44-46)
Earlier in the poem, the Duke says that he rarely reveals the painting for anyone. It is covered by a curtain. Now, the Duke can have the Duchess' smile all to himself. The painting, an object, is the way the Duke wanted the Duchess to be while she was alive: framed, still, obedient, giving smiles/affection only to him. Thus, the painting is a metaphor for the Duke's idea of ideal behavior for a Duchess. The painting is an object, a possession. The Duke objectified his wife in life and in death. The painting is not a memorial of his late wife; it is a metaphor for the Duke's insecure, jealous, and controlling behavior.