"to blindside" = "to present a difficult. emotional or awkward situation without warning": Ex.: "Mom broke down crying when I told her I had joined the Army. I think I blindsided her." You can blindside someone else (e.g., the example above), or you can "be blindsided BY" someone or something else (another person or thing caused a sudden situation that was difficult): Ex.: "Mrs. Jones was blindsided by her son's decision to join the army." The appearance of "blindsided ... WITH" in your sentence is a matter of sentence structure. The main sentence is: "I didn't mean to blindside you." The phrase "with Peter" means one of two things: 1. "along with Peter" That is, Peter was also responsible for blindsiding you. It could be understood this way: "Hey, PETER AND I didn't mean to blindside you." 2. "with the situation caused by Peter's presence" That is, bringing Peter along WAS the awkward or difficult situation (e.g., if a girl brought her friend's ex-boyfriend, Peter, to a party). It could be understood this way: "Hey, I didn't mean to blindside you BY BRINGING PETER."

blind-side or blind·side (blndsd) tr.v. blind-sid·ed or blind·sid·ed, blind-sid·ing or blind·sid·ing, blind-sides or blind·sides 1. To hit or attack on or from the blind side. 2. To catch or take unawares, especially with harmful or detrimental results: "The recent recession, with its wave of corporate cost-cutting, blind-sided many lawyers"