Many people raise them together with no problems. You should check with you local AG department to see if blackhead has been reported in your area. If it has not been them most likely it's not a going to be a problem. Generally Black head is most common in Warm wet climates. Most poultry deises come from guest visiting you, or other live birds, so if it's not in your area chance are your birds won't get it, but it's always a possibility. In any case you must always monitor your flock for health .
In our case the closest Chicken area is about 8 feet from the turkey pen. We don't house chickens and turkeys together because Turkey's have this thing about there space, and anything but a turkey coming closer then a foot or so. If Turkey's and Chickens are forced to stay together they do get a long. They do sort of intermingle when they free range.
You shoudl also look i the Turkey forum as there is lots of information there.
Tom