Amstel do have a dark beer, I was drinking it in Dortmund a couple of years ago it is very similar to Hieneken Old Brown. Very pleasant taste and not to high in the alcohol stakes. In Pamporovo this year I tried a dark beer which was quite similar made by, is it Kalamitza, one of the Bulgarian breweries. Perhaps some of the regulars on here might know the right name.
Walk down from town to the fountains, carry on past the Eidelweiss, past another couple of hotels (one called the Zodiac I think?) and you arrive at Kalina ... probably about 15 mins walk from the main drag.
Hey my friends, there some possiblities of what you drunk, for me this sounds as you had a Stolichno Bock Beer, draught one. Kamenitza tjemno(dark) is possible too, but only bottled and brewed in the winter, may be it was a Shumensko Tscherno(red). Or an Ledenika Tjemno.
But the Stolichno is the sweetest and strongest from all, sometimes you can get it in red cans. Good try in Burgas in the Schwejk Bier Bar one of the first bars at the main tourist street in Burgas.