Blackberry Germination:
1) Use a thermos to soak seeds in very warm water for 24hrs.
2) Cold stratify by placing seeds into a zip lock bag of moist peat moss or vermiculite, and place inside the fridge for 90 days. This will break the seeds dormancy and will allow them to germinate. Periodically check to make sure they do not dry out. Use a spray bottle to mist them so they're not overly wet as that can cause the small seeds to rot.
3) Plant seeds no more than 1/8" deep into a sterile seed starting mix. Keep "slightly moist" by misting with a spray bottle.
4) Use a heated propagator mat to keep the soil at 75 degrees F. during the day. Turn off the at night to drop the temperature to 68 degrees or less. This creates alternating temperatures which mimics their natural conditions in the wild which greatly improves germination rates.
5) Expect germination within 2-3 weeks.