Serbia

  
An Eastern European country at the heart of the Balkans, the core of what used to be Yugoslavia, before all the other members of the federation broke away, leaving rump Serbia landlocked, bordered by Romania and Bulgaria to the east, Hungary to the north, and all those newly independent former Yugoslav states (except Slovenia) to the west and south.
 
For the dark tourist, Belgrade is the main focus point too. It is also easily reached by plane, train or road, even river (Belgrade is one of the four capital cities on the banks of the Danube river). 
   
  
Around the turn of the millennium, Serbia was widely considered Europe's big bad pariah state. It had fought against those "break-away" countries seeking independence in the early to mid 1990s, amidst a whirlpool of rekindled ethnic confrontation. Serbia under President Slobodan Milosevic, apparently tried to sustain some out-of-proportion "Great Serbian" ambitions. Thus Milosevic's government supported the Serbian side in the conflicts in Croatia and Bosnia & Herzegovina, including war crimes that were thought no longer possible in the middle of Europe. The terms "ethnic cleansing" and "rape camp" were amongst the worst that became high-profile expressions in the media at that time.
 
After the particularly nasty parts of these Balkan wars in Bosnia (see e.g. Srebrenica and Sarajevo) were ended with Western intervention and a peace deal brokered at Dayton, USA, Serbia had become largely isolated.
 
Things got worse again with the outbreak of the conflict in Kosovo in the late 1990s, culminating in the extensive – and controversial – bombing campaigns of Serbia by NATO in 1999, including targets in the capital Belgrade. The whole story was (and still is) an incredibly complex matter, diplomatically, politically and morally, and I won't go into any depth here or take sides. Suffice it to say that the Serbs felt hard done by but had to back down in the end.
 
Milosevic himself was ousted by Serbian protesters in October 2000 and later arrested and tried for war crimes at a special Criminal Tribunal in The Hague, Netherlands (but died before being sentenced). The status of volatile Kosovo, meanwhile, remains somewhat unresolved to this day, although the situation seems fairly stabilized for now. The country's self-declared independence of 2008 continues to be a bone of contention, especially between Russia and the West (also in context of Russia's war with Georgia over South Ossetia).  
 
In the meantime Serbia has embarked on a course of integration with Europe, even seeking EU membership, though NATO membership, given the country's recent history, still seems very unlikely. UPDATE June 2022: given that Serbia has largely taken Russia's side in that country's war against Ukraine, EU integration seems further away than ever for Serbia. 
  
However, the most wanted war criminals from the Yugoslav wars (Karadzic and Mladic) have meanwhile been arrested and extradited to The Hague too. So acceptance of Serbia back into the "international community" was on the up for a while. At the same time, travel to the country has become a much hyped "insider's tip" in recent years, in particular the capital city, Belgrade.
 
The only specific dark tourism sites outside Belgrade that I am currently aware of are in or near Nis, Serbia's third-largest city. The famous "Skull Tower" may qualify, featuring walls into which are set numerous skulls of Serbian rebels killed by the Ottoman Turks in the early 19th century, though strictly speaking it falls a bit out of the time-frame for the concept of dark tourism.
 
On the other hand, Nis also has a fairly well-preserved site of a WWII concentration camp (known in Serbian as "Crveni Krst" camp, or as "Lager Nis" in German), just north of the centre, as well as the Bubanj Memorial Park on the southern outskirts that commemorates massacres of over 10,000 camp inmates, mostly Jews and Roma, but also other citizens from Serbia. So far, however, I have not been able to visit these sites myself. But I will report back once I have, and then make proper separate entries for them. And if anyone can give me further pointers for Serbia, I'd gladly follow them up (please contact me).