They’re different - when I say “brush” my tongue stays in the middle of my mouth against the bottom and the back spreads slightly on the r, when I say “blush” it touches the top of my teeth before flicking down on the u

They’re slightly different phonemes, if you don’t hear it naturally it’s difficult. But if you focus hard enough on the details you can eventually learn to hear it - you have to listen intently to the comparison over and over until you can hear the difference. From there, you can practice to make the sound yourself