I agree, there's actually no difference b/n the term dongle and adapter.
One should note that for Bluetooth to work properly a Bluetooth stack will need to be implemented. Most "commercial" USB Bluetooth devices/dongles implement the stack in the host software! This means you need to connect it to a Mac or PC and install the necessary drivers for it to work correctly!
Having a USB doesn't imply you'll be able to talk properly with the USB device. As hazzer123 pointed out, you have to consider if your MCU has host capabilities. Also you have to consider if you have a USB stack that is implementable for your MCU. Maybe your "USB as UART" setup would work, but I have no experience on that.
What one will want is a Bluetooth device/dongle/module that internally implements a Bluetooth stack in firmware (in the device itself) and provides us with a simple interface (UART is good, simple and very easy). Important is Serial Port Profile (SPP). This profile emulates a serial port over the Bluetooth connection. hazzer123's device suggestions seems to fit the bill properly.