I agree that umbrellas should not be used for low key work. (I shoot a lot of low key too.)
Think of an umbrella as a shotgun compared to a softbox or grid being a rifle. The umbrella throws light everywhere and is hard to control. Most likely your black background is being lit by your umbrella fill.
I have one of the Paul Buff snoots. If you do too then you are already using a grid. That is because the Buff snoot is simply a grid with a round piece of metal on the front constricting the light to a small circle. Frankly, I would never use a snoot for portrait work as they are much too directional. I find the grids work better. My suggestion would be to buy the set of 4 grids for around $100 USD and get to know which ones you like. Then buy more of that one.
The big problem I have with low key in my small studio (see my website section "Products" then move down to the last photo) is spill from the lights. So next to each light I have a second light stand holding a 20x30 inch black Foamcore panel. These were purchased for about $7 USD each from Office Depot.
Often I will bounce a strobe off one of the white walls of my studio but I will put a piece of black Foamcore on each side of the strobe. This way the bounced light will not hit my backdrop.
If you have a softbox and on the front edge there are strips of Velco sewn on you are in luck. Take a look on eBay for "honeycomb panels" or "grids" for your soft box. These are a set of long thin black strips connected with fabric. When attached to the front of your softbox they restrict the light coming out to a narrow angle. There are two designs. One is a set of strips which are all parallel. (Think of Venetian blinds you might have seen in people's homes.) The other design is made with strips running in both directions so you end up with a grid. This design restricts your light output even more to be quite directional. The one I use is the first type with just the strips running in one direction: |||||||||
Take a look at my website under the section "Headshots" or "Portraits/Adult". There you will see a low key portrait of Marek Kedzierski, a theatrical director from Poland who was in Atlanta for a couple weeks. I created his portrait with two of my 800WS Alien Bees. Each had a grid over the reflector. (I don't remember which grids.) One light was about 20 feet behind his right shoulder (camera left) just barely illuminating the back of his head and his right ear to keep them from merging with the black area behind him. The second light was actually aimed at a white piece of theatrical fabric which was right next to my camera. Only the edge of the grid-restricted light was hitting his left ear, the majority of the light was reflecting back to illuminate his face. This sort of lighting is quite contrasty and looks like spot lights used by Hollywood photographers of the 1930s (a look I was trying to emulate). I used Irfan View to convert the RAW file to monochrome (B&W) then forced the image to be red. This simulates the look of orthochromatic film which was what they had in the 1930s. Finally I used Irfan View to adjust contrast and density to what you see on the website. This technique works for men with character lines but I would NEVER do it to a woman as every line, crack and mark on her skin would show up!
Hope this helps you.
Terry Thomas...
the photographer
Atlanta, Georgia USA
www.TerryThomasPhotos.com