dnd 5e – How to balance Bless a weapon through prayer

I am not an experienced player and I need help. My character is a level 3 cleric who follows the domain of deception. The DM has stated that with a prayer session long enough in a sanctuary I can bless the weapons without using a spell. However, the DM wants you to find out the effect of the blessing and how to balance it. So I'm thinking that the blessing would act in a similar way to the blessing of the spell or it would do additional damage to the undead it creates. So I'm not sure if the weapon should be blessed for a certain time or if the blessing should be eliminated after x number of attacks with that weapon. Also, would it scale (adding a d4) with the favor I have with my god? The DM is very inexperienced and is using many house rules.