Heya.
A magical weapon is already an a Masterwork weapon. Thus its just "+1 dagger". (just being a know-it-all)
0. The Bleed effect isnt too impact-full IMO, so I won't take it into consideration.
1. The silence is a 2nd level spell so a minimum of level 3. Its and AOE with a save so take that into consideration.
2. The ambush feat from Complete Scoundrel, Throat Punch requires +3d6 sneak attack (Rogue level 5) and Imp. Unarmed Strike. This needs to be a sneak attack so, also take that into consideration.
3. IMO a +1 weapon should be given to a player around level 3-4.
Short answer: I would say at level 3-5 would be appropriate.
* The silence effect is for niche targets, that are susceptible to poisons and are casters who require verbals spell components (not taking into account guards who need to be silenced so they don't call for help).
* The Bleed effect is also for living beings only, and it does very little damage and scales poorly.
I have an idea, I'll just put it out here.
* Make the dagger(s) a +1, as is.
* Give it to the player at level 3.
* Now here comes the tricky part, get Complete Scoundrel and give the dagger(s) all the Ambush Feats in the book. This will scale with the rogue, since the feats require +xd6 sneak attacks to be trade for effects. Remove the ones that you don't like or don't fit, like Mind Drain (only against creatures with Psionics).
* Name the daggers "Tricks of the Trade".
* This way you don't have to invent rules for the effects, the daggers progress with the character, thematically its perfect for a rogue, and just like your effects, these only work on living beings.