In conkyrc, above the TEXT line:
# Lua
lua_load ~/scripts/draw_bg.lua
lua_draw_hook_pre draw_bg
In the lua script, the lines:
bg_colour=0x000000
bg_alpha=0.6
adjust color and transparency. I forget if the higher alpha is darker or lighter.
Also, the line
corner_r=0
Adjusts corner rounding radius, with 0 being none. I think the default was 35 when I got it. I also just noticed the script has some explanations in the comments, so most of what I just told you is probably superfluous, lol
One other thing, if you use ${image} in your conkyrc, I beleive it is drawn BEHIND the lua transparency. But I'm not 100% sure. I just have one image I was messing with show up a whole lot dimmer than it should be.