Blank verse, of course, is regular meter with no rhyme. The most common blank verse in English is iambic pentameter, famously used in William Shakespeare's plays and also employed in this poem by William Wordsworth.
As I understand your question, you want to know where the iambic pentameter is not adhered to rigidly. I'll glady point out a few lines to get you started. I think you'll be able to take it from there.
Let me first show the iambic pentameter of line 3, capitalizing each stressed syllable (rather than using the usual tick mark, as I can't figure out how to make that work on this site):
These WAters, ROlling FROM their MOUNtain-SPRINGS
This line shows what I think you mean by "strict blank verse": there's a very strong iambic meter; that is to say, there is a reliable alternation of unstressed and stressed syllables for a total of 10 syllables in the line. (When I scan a poem, I usually skip the first line or two, as the patterns in opening lines of poems don't always immediately stand out for me.)
The iambic pattern from line 3 isn't adhered to strictly in line 7 because of the two unstressed syllables side-by-side ("of more"):
THOUGHTS of more DEEP seCLUsion; AND conNECT
Line 10 similarly has a little play in the meter. The phrase "under this dark sycamore" can be read aloud in a number of different ways, but the two small words before the tree's name through off the meter just a little for me.
Scanning a poem takes a little time and occasionally shows that a line can be read several ways. When possible, consider choosing the most natural pattern of stresses, coming as close as you can to how you would say that phrase yourself. Be aware, of course, that dialects of English may use the same words or phrases but different stress patterns (e.g. the American and British pronunciations often differ in words such as "secretary" and "military").