Getty / Facebook @Brittney Jones Composite

Kelly Clarkson 's ex-husband Brandon Blackstock was in a relationship when he died of melanoma ... and she's a former assistant to Kelly.

According to his obituary ... Blackstock was living with Brittney Marie Jones in Montana -- a "loving partner" who was with him until his untimely death. A source with knowledge confirmed to TMZ ... Brittney was a production assistant for Kelly from 2016 to 2020.

According to her LinkedIn she was then Blackstock's executive assistant while he was the artist manager of Starstruck Entertainment -- before he rebranded the company to V Bar V Cattle Co., after the couple's divorce. Document Launch Doc buttefuneralhome.com

TMZ reported last week Brandon sadly lost his battle with melanoma, after

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