John Milton's "On His Blindness" is a Petrarchan sonnet. These are also called Italian sonnets; they're the same thing.
When you have to determine what kind of sonnet something is, the answer is usually either Petrarchan/Italian, like it is with this one, or Spenserian, or Shakespearean. And it's usually very easy to tell which is which, because you just have to look at the end of each line and see what the pattern of rhyming words is. More information is here.
You can tell that this one is a Petrarchan one because it has a rhyme scheme of ABBAABBA for the first eight lines, which automatically means it's a Petrarchan sonnet.
When we're talking about the rhyme scheme, any Petrarchan sonnet's fourteen lines are organized into two big clumps, each with its own little pattern of rhyming words: the first eight lines (called the octave) and the last six lines (called the sestet).
In the octave (from "When I consider" all the way through "patience to prevent") you see the rhyme scheme "ABBAABBA." It's the same for every Petrarchan sonnet. That scheme plays out like this in our specific poem; notice how all the "A" lines rhyme with each other, and all the "B" lines rhyme with each other:
You can see why those rhymes can be labeled "ABBAABBA."
Next, in a Petrarchan sonnet's sestet (the second of the two "clumps" of lines, which consists of the last six lines) the rhyme scheme is any one of these options:
For "On His Blindness," the sestet starts with "That murmur, soon replies" and goes until the end of the poem. The rhyme scheme of the sestet happens to be that third one I just listed, CDECDE:
To sum that up, if a sonnet's first eight lines follw the rhyme scheme ABBAABBA, then it is a Petrarchan (or Italian) sonnet.