Poison damage has 2 components; damage ticks and over time. Your resistances affect the first one, so 85% res mean you receive 15% of the damage ticks. Poison length reduction affects the second one. Either ways, the only way to be poison immuned is via shrine.

Cold damage also has 2 components, but more straightforward; damage and freeze length. Resistances affect the first one, cbf/half freeze duration affects the second. Cbf will negate any freeze length.

Cbf will prevent you from being frozen by duriels cold attacks, but will NOT prevent you from being slowed down by his holy freeze. Holy freeze's slow effects falls under a different mechanic, "slow target". Cbf has no effect on this.