Blended fabrics are a mixture of synthetic and treated yarns. Mixtures are available at different levels from 50 - 50 mixes to 80 - 20 synthetic to treated.
The main advantage to blended fabrics is that you can gain the advantages of both synthetic and treated fabrics whilst minimising the disadvantages that each fabric has.
Naturally a blended fabric will never achieve the same level of performance that a synthetic or treated fabric may offer against a particular hazard. However, on a site that has different types of hazards, a blended fabric may offer a good compromise.
The cost, wear and performance of a blended fabric is very much dependant upon the mix level so it is very hard to provide general comments about these issues.