You should always put the double stick velcro directly to your battery tray and battery. Then use the velcro strap to make sure it doesnt vibrate lose. If you only used the strap and no velcro directly between the battery tray and battery...this is where you messed up first.
Your second mistake was running the heli for 15 minute flights. While I dont own the Blade 450...I do have a clone TREX 450 and the most i fly a pack is 6-7 minutes and that brings the voltage down to storage level (appox 3.7-3.8 volts per cell). While you can take them down to 3.3 volts per cell...its not a good idea.
So...either the battery vibrated around and came in contact with your motor...causing it to fail. OR, you depleted the battery too low and over several cycles on the charger....it damaged it, resulting in a catostrophic failure of the battery.
This can and WILL eventually happen when you deplete a lipo too low. You should NEVER take more than 20% of your battery capacity. So a fully charged lipo will be 4.20 volts per cell. When you deplete 20% of that...you would have around 3.8 volts per cell.
Using the above method will make your batteries last the longest and keep you and your model the safest.
Blade definitely is a good company based on them giving you a replacement with pretty much no questions asked. Because I feel that it was more user error than faulty product that caused your issue.
Please dont think i am throwing you under the bus with this post. Its just it sounds like you were uneducated about the way to mount your battery and charge/care for them. So now you know and you should have nothing but good results from this point forward.
goodluck.