Just make your attack rolls with bless look like JustinCase did:
Attack: [roll0] + bless: [roll1]
And crits occur on a nat 20 and nat 20s always hit. You double the dice (all of them). So [MENTION=6776182]JustinCase[/MENTION], roll a 1d4 and a 1d6...this is why giving rogues advantage is a huge thing.