I made it out of the underdark with the silver dragons blood (thanks to Memnoch) but what do I do with it?

SPOILER>>>>>>>>>>
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
You take it to the human flesh armor maker in Umar Hills. You need to have completed the Tanner Quest. Scroll down for his name.
.
.
.
.
.
.
.
.
.
.
.
.
.
Darcin Cole.

and you get?

If I remember correctly: Human Flesh armor +5 (only usable by evil)
Does anyone know if your rep goes down from killing the silver dragon (i think so) and if so what is the cheat code for her blood?

It probably would - you're killing a noble creature.

Silver Dragon Blood - MISC8H

I was told I have to be finished with the tanner quest.
I found tanner shop but no quest that went with it.
how do I get the tanner quest??

Depends on which part of the quest you're talking about.
There's a skinner murder quest in the Bridge district I'm sure you've done.
There's a continuation in Trademeet - Skin Dancers.
There's one in Umar Hills which is minor and only really for evil characters.