Bluegrass is a lawn grass, like Kentucky Blue. It only gets 8 " tall or so and is not usually found in pasture hay except by accident. It won't hurt the horses, but it isn't a "feed" type grass.
You must be referring to the Blue Stem grasses. There is a Little Blue Stem and a Big Blue Stem that are two different species, but the only real difference is how high they grow.
Big Blue Stem grasses are the ones that grow about waist high. It is an ideal grass for pastures and for baling. It has excellant nutritional value in forage or in hay. It is a warm season grass so it doesn't start to green up and mature until a little later in the season, around here in Nebraska that means around May. It is very common, it is a native grass in this area and probably is one of the main grasses in what people call Prairie grass. Frequently you will find Big Blue Stem mixed with Brome as the brome is a cool weather grass and so starts to green up far earlier than the Blue Stem. Between the two of them you can have a green pasture for longer. Right when the brome goes dormant, the bluestem kicks in. Then in the fall the Blue stem starts of back off and the Brome kicks in again. It is one of the main forage grasses around here and fed to just about everything with success. We put up about 100 acres of it each year to feed our horses.
Right now it is selling for about $2.50 per bale. I wish I hadn't sold all our extra! I'd be sending it your way! I hope this info helps.
Ps. Around here the horses seem to like it better than the brome just because the leaves are wider. Nutritionally they are pretty similar. Our Alalfa is selling for $3.50 per bale. I just so wish I could share all this with those of you paying $8.00 a bale!