We have many, many chiggers here in MO. By the time they begin to itch, the chigger itself is long gone--the itch is a reaction to the chemicals in the the nasty little buggers saliva.
Prevention is key. Keep lawns mowed, stay out of high weeds and grass, especially if the temp is over 80 degrees. If you do decide to go blackberry picking, use DEET repellent before you go out, and take a shower the instant you come back home, with lots of soap.
If you can't get to a shower, you can try rubbing your skin and clothing quite firmly, as if you were drying off after a shower. Chiggers have soft bodies, and rubbing crushes them.
Benadryl tablets and cortisone creme work for the itching. Sometimes people get soooo many they get a weepy red rash, like poison ivy. Then it is time for the doc and some prescription "big guns".
If you think the above sounds like the voice of experience, you are right.