Your BLU Tank 2.4 Xtreme features a MicroSD memory card slot. If your personal computer is compatible with that type of card (Most laptops are so-equipped), you should be able to save a music file (The BLU Tank Xtreme 2.4 supports these formats: Yes) from your computer to a MicroSD memory card, and then, from the memory card to your phone. Your manual should tell you how to assign the file as your phone's main ringtone.
The Tank Xtreme 2-4 is equipped with Bluetooth version 3.0, which lets you connect to your computer wirelessly. Just pair phone and computer together and drag and drop your music files from your computer to your BLU Tank Xtreme 2.4.
If all of the above methods do not work with your BLU Tank Xtreme 2.4 or if you prefer not to buy new accessories, you can still use one of the numerous free '3G Uploader' services available online. A search on Katapulco should retrieve plenty of them. 3G uploaders work as follows: Save your own music on their web site and they will provide a temporary URL. Navigating to that URL with your BLU Tank Xtreme 2.4's browser should download the free ringtone.
Here are some of the free ringtones (BLU Tank Xtreme 2.4-compatible) offered here at MartinsRingtones.com:
Free Horror MIDI Ringtones Free Ghost Ringtones Free Ringtones-a-gogo Free Techno Ringtones Listen To All Our Free Polyphonic Ringtones