AFAIK only a "USB Host" port can send Midi. If your Boss switcher has a USB Host port, like e.g. the Morningstar MC6 Pro has, I don't know, but a delay or other client device will most certainly only have a "USB Device" port (a device port can only communicate with a host, not with another device - there is no daisy-chaining in USB).
The other issue here is of course that the actual encoding/encapulation of data is completely different between a "raw" Midi stream coming out of a DIN port and whatever highly involved encapsulation is used for Midi via USB. A simple adapter cable is not solving this.
The usual solutions include: a) using a Midi controller with a dedicated USB host port (e.g. Morningstar MC6 Pro, Disaster Area dmc.micro pro or dmc 6), b) direct connection to a computer, and c) adding a dedicated USB Midi Host adapter to the board (e.g. Miditech or Kenton, Kenton if you want it to last longer than the warranty).