"Do they need to be 100% wool or are the 70% wool blankets ok?"

It might depend on what the other 30% is, and they often don't say. Is it cotton, which will increase drying time? Is it polyester (plastic) which will burn?

Another question to ask (presuming you're asking someone who knows something, which is doubtful when it comes to cheap blankets) is, is all the wool in the blanket virgin wool (never previously made into fabric) or recycled wool? Even virgin wool has different grades of quality, so there's a great difference between wool for underwear and wool for carpeting.

Many of the cheap wool blankets (even 100%) are made from recycled wool, which has been torn or cut apart and respun into 'new' fabrics with shorter strands and blunt ends. Most of the recycled wool blankets I have seen are dense and tend to be itchy. Compare them to high-quality ($200-300) blankets, and you can tell the difference immediately.

Some wool blankets call for dry cleaning, which isn't really necessary. Washing with human hair shampoo, with a human hair conditioner in the rinse does very well, and they can be dried in a dryer, but use low heat and short cycles, as they dry quickly.

True wool allergies are quite rare. People who say they're allergic to wool are most likely to be sensitive to coarse, recycled wool.

Sue