Blancco Mobile Diagnostics and Erasure is not able to fetch IMEI for Windows Phone devices. The protocol that BMDE uses in erasure doesn't allow fetching the IMEI code. To be able to fetch IMEI code, it would be required to install an application on the device, which would make the erasure process highly inefficient.
The erasure report shows the "internal serial number" of the device. It is unique to the device, but not the same as the "commercial serial number" that you can find if you remove the device back cover and look under the battery.
The best way to match the device and erasure report is the following: