Picture a driver with both hands on the wheel, eyes forward, mirrors covering most of the road but not all of it. There is one patch beside and behind the car that nothing shows. That patch is the blindside, and the word is built out of exactly those two pieces: blind and side. Everything else the word does starts from that patch. Something comes out of it and hits the car. Something comes out of it and hits a person. And in the meaning people use most, nothing physical arrives at all: the news or the question comes from the direction you were not watching, and you had no chance to get ready.
From the word’s history
Wiktionary gives the formation plainly: blind plus side. The word is transparent, and the pieces still carry the whole picture.
依 Wiktionary，這個字就是 blind 加 side 組成的。結構很透明，兩個零件到今天還完整撐著整個畫面。
Wiktionary entry for blindside, etymology_text supplied in this packet (source_ref kaikki:3f6f69a190036ae4:886641685:10851)
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