A blazon in literature is a word that normally refers to a lyric poem that focuses on the speaker's beloved and where his or her virtues and beauty are listed one by one. If you want to see another example of a poem where this form is subverted ironically, look at Andrew Marvell's "To His Coy Mistress," which features a parody of the device. However, in this sonnet, we can see that Shakespeare likewise subverts this form. Instead of using a blazon to focus on his lover's beauty and virtue, he uses it instead to focus on her many faults and the way that she is all too human. Consider the following example:
I have seen roses damask'd, red and white,
But no such roses see I in her cheeks;
And in some perfumes is there more delight
Than in the breath that from my mistress reeks.
This is rather a humorous example as we are made to catch a whiff of the terrible bad breath of the speaker's mistress. However, as the ending of this sonnet indicates, what Shakespeare is really doing is using the blazon ironically to poke fun at other poets who describe their objects of affection in such terms that rob them of all humanity and make them appear to be gods and goddesses:
And yet, by heaven, I think my love as rare
As any she belied with false compare.
We can see therefore that Shakespeare uses the blazon device to establish the down-to-earth reality of his mistress, and to point out that his love is just "as rare" as any other loves that rely on wildly exaggerating the beauty of the beloved. Reality, and acceptance of the other's weakness and faults, Shakespeare seems to be saying, makes love even more profound than relationships that rely on an absence of reality.