Brandon Blackstock’s relationship with ex-wife Kelly Clarkson’s former assistant Brittney Marie Jones was reportedly “the icing on the cake of the disappointment” the singer felt over the dissolution of their marriage.

A source told the Daily Mail Thursday that Blackstock — who died last Thursday at the age of 48 following a private, three-year battle with melanoma — “chose to move on with someone Kelly knew and trusted,” which “just adds to the person he ended up being to her.”

The source described Blackstock, who shared two children with Clarkson and two with his first wife, as “a great father,” but not “a great husband.”

The source made sure to stress that the “American Idol” Season 1 winner “doesn’t blame” Jones, who lived with Blackstock in Montana before his death, and w

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