Use of detergent is going to ruin the blanket's waterproofing, just saying.
If you use a detergent specially formulated for blankets, or if this is a stable blanket that you're talking about, you should be ok and you should ignore this. Haha
Another tip I've heard is to use a front-loading washer, apparently they clean better than a top-loader.
We have a top-loader at my house (yay for appliances from the 1980's!) and I tried washing a blanket in it and it was a complete failure. I ended up running the blanket through at least 4 times and it was STILL dirty.
I usually just hose off Lacey's blankets in the front yard (or I drape them over the side of our deck) and scrub at them with one of those super hard wooden brushes. It's not super classy and it doesn't do an A+ job but it does do a B+ job so I'm ok with it. Haha