There are more than 1 issue and depending on what style you want. Seeds can add bitterness and excessive tannins from some fruit so leaving them in to long might make it take forever to age out and be good. The spoilage thing can be a problem if you are not stirring or if you dont put on an airlock in time. The other thing is that the fruit will just continue to fall apart after the flavors have been extracted for no gain, just making more particles that need to be cleared. Some people expect their melomel to taste exactly like the fruit, so adding it to the secondary after fermentation is finished is just flavoring the mead with fruit. Others prefer to have a more complex fully fermented taste and put the fruit in the primary until the primary ferment is done or remove it early to avoid excessive tannins or bitterness. We sometimes as mentioned above like to do both if we want to boost a little more fruit flavor, or ever add some strawberries for a few days to any fruit wine to give it a little more nose. So you see its a hard question to answer, depends on the fruit, they style you want and whether you dont mind more sediment in your wine or have to leave it in longer due to a trip out of town etc. You might due your experiment as you noted, but that would pretty much be for just blueberries, you would need to repeat it for most different fruits, like peaches, or elders, or blackberries, each is unique, this aint simple grape winemaking where all you have to do is squish some grapes and everything is easy. WVMJ