Chocolate Twang Aftertaste
At a guess, you're tasting the chocolate.

Plain chocolate can actually have a lot of different flavor notes, depending on the bean type, origin, batch, and year.  A fruity taste is quite likely the result of working with a single-origin batch of beans that had that characteristic.  Other tastes can be mellow or sharp, reminiscent of different scents or flavors or kinds of sweetness that are actually not added to the chocolate bar - caramel, or tobacco, or marshmallow, or various fruits, spices, herbs.  Various kinds of fruity tastes are quite common - you can look up descriptions of types of cocoa beans for sale by batch or origin, or single-origin chocolates, and they will usually describe what typical notes might be found in the beans, growing area, or batch that they used.  

Of course, some kinds of beans don't have that varied flavor, being more single-note.  Mixing several batches of beans can also lose the extra flavors, since instead of a few notes working together strongly, the much more varied and less concentrated extra tastes get kinda mixed together into a generic sweetness.  Big chocolate makers will likely deliberately blend different batches of chocolate together to get specific and reliable tastes in their products.  And those kinds of scent undertones are also often lost the more processed or diluted the chocolate is, with the addition of sugar or milk powder to help mask them, which is where the "typical" chocolate taste comes from, and why you don't get fruity notes until you hit very dark, pure chocolate.

If you would like to avoid the fruity notes, then you should perhaps avoid single-origin chocolates, or else specifically look for descriptors that match what you're looking for (smooth, intense, deep notes, something like that), and avoid what you don't want (fruity, tangy, floral, light).  You can also track which companies blend their chocolates the way you like, for pure dark chocolate (they will want consistent products, you understand) - though I have no knowledge of how the companies stack up against each other in this fashion, you may be stuck with trial and error.