RE: Windows 7 key
Windows Blue = Windows 8.1
P = Pro
Looks like your system shipped with Windows 8.1 Pro with Downgrade rights to Windows 7 Pro.
I advise clean installing Windows 10 Pro directly. Windows 10 TH2 installation media will automatically pick up the Windows 8.1 Pro from the UEFI BIOS and activate when online.
Tech Enthusiast and Author of the Unofficial Dell Windows Reinstallation Guide