The father of Grand Duke Nikolay Konstantinovich's morganatic spouse, Nadezhda Alexandrovna Dreyer was Chief of Police in Orenburg Alexander Gustavovich (von) Dreyer and her mother was Sophia Ivanovna Opanovskaya, according to the Russian Wikipedia. From
this database of foreigners in pre-Revolutionary Russia we see that her father was a Baltic German.
This is her grandfather Gustav von Dreyer, apparantly an auditor and estate owner in the Narva area.
The name of the estate, Slepow Koniec / Slepov Konez / Slepov Konyets, seems elusive, though. It's probably not in Estonia, as it doesn't sound like (Germanized or Russified) Estonian at all. It's probably in the St. Petersburg Governate. In Russian Слепов конец means something like End of the Blind. One the Russian Wikipedia definitions of конец is:
устаревшая единица территориального деления населённого пункта, охватывающая одну или несколько улиц (ремесленный конец, рыбацкий конец). Как правило, такое деление возникало в небольших посёлках и городках и позже становилось тем или иным районом.
=
an old unit of territorial division of settlements, covering one or more streets (craftsmen's end, fishermen's end). As a rule, such a division arise in settlements and small towns, and later became this or that district.
So a division of a village or small town called "Blind End", meaning it was inhabited by blind people or was a dead end would seem possible. There are two villages in Leningradskaya Oblast called simply Конец today, both situated southeast and east of Lake Ladoga, but it doesnt look like it's any of them.