How we cite our quotes: (line)
Quote #1
Sir, ’twas not
Her husband’s presence only, called that spot
Of joy into the Duchess’ cheek (13-15)
The Duke is offended that the Duchess would take pleasure in anything other than him. Notice that the way she shows her pleasure is involuntary, (i.e., a blush counts as showing pleasure), but the Duke describes it as though it were a stain or taint, a "spot of joy."
Quote #2
perhaps
Frà Pandolf chanced to say, "Her mantle laps
Over my lady’s wrist too much," or "Paint
Must never hope to reproduce the faint
Half-flush that dies along her throat" (15-19)
The Duke’s jealous fantasies are very elaborate – he’s imagined in detail the kind of compliments that the painter might have paid to the Duchess, and the coy way that she might have responded. It’s important to remember that, as far as we know, this could all be in his head. There’s no evidence in the poem that the painter said these things or that the Duchess blushed in response.
Quote #3
such stuff
Was courtesy, she thought, and cause enough
For calling up that spot of joy. (19-21)
The Duke seems to believe that the Duchess chooses to blush or react to compliments and gifts. He describes her as "calling up" her blushes, instead of experiencing them as an involuntary reaction. As readers, we know that she probably isn’t blushing intentionally, and the Duke’s jealousy is illogical.