Lines 7-12 Summary

Get out the microscope, because we’re going through this poem line-by-line.

At thy return my blushing was not small,
My rambling brat (in print) should mother call.

Now it seems this little book was eventually returned to the speaker, who blushed quite a bit when she saw it. (Her “blushing was not small,” which is an understated way of saying she blushed a lot.)
She probably blushed because this book came back in the form of an actual, bound, published book—not just as a bunch of pages stapled together or something.
Actually, her blushing might also stem from the fact that her name is now in print. That seems to be the gist of line 8.
There, the book is referred to as a “rambling brat” (compare this to the first line’s “ill-formed offspring”) that now, in print, calls her (the author), “mother.”
In other words, this collection of crappy, ill-formed, rambling poems now publicly declares in print that its mother is none other than Anne Bradstreet.
As in the first few lines of the poem, we are struck by the extreme degree of self-deprecation. Take it easy, Anne, the poems aren’t that bad.

I cast thee by as one unfit for light,
The visage was so irksome in my sight,

Here we get more trash-talking. This time, the “rambling brat” of a book is cast aside by the speaker because it is “unfit for light” (i.e., an abomination, a monster, something that needs to be hidden in the attic).
This book is “unfit for light” because its “visage,” or appearance (or face, or countenance, or what have you) was unbelievably “irksome” to the speaker.
Note: the book isn’t not actually a monster or a person with an actual “visage,” just as it is not actually a brat (like that screaming kid you saw at Walmart earlier). The speaker keeps personifying her book, probably to make it seem like a much uglier, trashier piece of junk than it really is.

Yet being mine own, at length affection would
Thy blemishes amend, if so I could.

Oh wait a second now, it looks like things are starting to change just a bit. In the same breath, the speaker now says, essentially, “well you’re a brat, and irksome, but you’re mine” (“yet being mine own”).
Since this book is hers, the speaker says, if it were possible she would eventually (“at length”) at least try to “amend” its blemishes (faults, mistakes, errors, its irksomeness or ill-formed-ness) out of some sense of affection.
Okay, technically “affection” would amend the blemishes, not the speaker. But this is essentially the same thing. It is merely a way of emphasizing that “affection” for one’s own “children” sometimes makes us do things.
Of course, the implication here is that the speaker is unable, or will be unable, to “amend” the faults of her ill-formed, irksome child of a book.
Well three cheers for the positive attitude, anyway.