No, you cannot upgrade the Horn of Blasting. However, you CAN upgrade the Horn of Valhalla.
Spoilers
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The Horn of Valhalla can be found in the Docks district, in a vase in the House next to the guy who sells fish. To upgrade it you'll need a Beljuril gem and a Diamond (you can find a Beljuril in the Umar Hills and I think also in Firkraag's Dungeon). To upgrade the Horn, go to the weaponseller in Waukeen's Promenade.
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Melusine, Archbabe of the OHF and the LH
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