I visited the citadel of Verdun this summer, and while visiting I witnessed some flags which I recognized all accept one, it was composed of three horizontal bands from top to bottom, black blue and white, and written in the center "HI". I searched a lot and I asked directly to the staff of the citadel and they told me that it was "a serbian flag but we don't know what the HI stands for" with no further explanation, and I can't find it on internet, I hope you'll help me, thanks.
The monogram you saw was actually Cyrillic - Н.I - or NI in Latin: it's the royal monogram of Prince (1860-1910) and King (1910-1918) Nicholas I of Montenegro. You've probably seen a variant flag of Montenegro with his monogram (posting the link as it's supposedly not easy to find this variant for non-Serbs). The first color was probably dark red and/or has darkened over age.
