Hello CozyBaker. I completely understand your concern, as a discolored and overly wet muffin is definitely not what you are aiming for.
The short answer is no, you should definitely not thaw your frozen berries before adding them to your batter. Thawing breaks down the fruit cell walls and releases a significant amount of juice. If you mix thawed berries into your batter, that excess moisture will streak everything purple and disrupt the moisture balance of the crumb, leading to a soggy texture.
To achieve bakery-quality muffins with vibrant, distinct pockets of fruit, here is the method I recommend following:
Keep the berries in the freezer until the very last second. You want them rock-hard when they meet the batter so they do not have time to release their juices.
Toss the frozen berries in a tablespoon of the flour from your recipe or a teaspoon of cornstarch before adding them. This light coating absorbs any surface frost and creates a barrier that keeps the berries suspended rather than sinking directly to the bottom of the tin.
Fold the berries into the batter gently using a silicone spatula. Limit yourself to just a few gentle turns of the bowl. Overmixing will cause the skin of the berries to rupture and bleed color.
Be prepared to extend your baking time slightly. Adding very cold fruit lowers the internal temperature of the batter, so your muffins will typically need an additional two to five minutes in the oven. Test doneness with a toothpick inserted into the cake crumb rather than directly into a berry.
I hope this explanation helps you get the exact texture and appearance you are looking for. Happy baking!