Anti tarnish finish or coating is quite common on some (not all) silver plated items. Some types of anti-tarnish will be removed by antiquing solutions (like LOS). Other kinds require a solvent. Acetone (dip in or wipe on with Q-tip) usually takes it off. Some experimentation is required.
As Luann says, if you just want a duller finish or a matte finish abrading the surface will do it. I like the super fine steel wool for that. All of Luann's suggestions work well too. Depends on what you have handy.
"What you have handy" reminds me of some old school methods of darkening the surface of findings. Again, some experimentation will be required before you commit a large batch of goods to one of these methods.
1) I used to produce a line of filigree and bead earrings where I hand antiqued the filigree myself. I used to put a batch of filigree findings in a rock tumbler, spray in some cheap black spray paint, add 5 or 6 drops of paint thinner and tumble them for a few hours. After tumbling I'd lay them out on old newspaper to dry. I'd end up slightly dulled highlights and a lot of dark in the crevices. It didn't really work on a smooth, high polished item as it was all highlight. But even on a shiny, smooth piece I'd get a little duller finish because a small amount of very thinned out black paint covered the whole item. Depending on your needs that might work for you.
2) There are a couple of kits on the market for antiquing of costume jewelry. I can't find the links right now, sorry! One website has a great how to antique with magic marker and acetone that gives an effect similar to my spray paint and thinner method. http://svallee.com Updated 12/3/2010 - this website not active.
Her site has a lot of information about various antiquing methods, specifically for costume jewelry. Using Tarn-X, baking it in the oven, magic marker, etc, etc. Spend a few minutes on her site and see if any will produce the effects you want.
Remember though, a plain surface like a head pin, many chain styles or a large domed piece does not have "low spots" to hold the antiquing. It can only change in color or become duller.