Because DHL will be providing courier service on behalf of blue dart.(if this is the case i.e, invoice is raised only for the courier service).
And another point to be considered is that there should not be double taxation i.e, if the blue dart company has already charged the service tax from its customer and that customers courier is handled by DHL then DHL should not charge service tax.
If this is not the case and DHL is not charging the said tax then it will be the responsibility of Blue Dart to deposit the said tax with the Govt. either it can collect from DHL or it should pay from its own pockets!