[Pursuivant]

Two thoughts:

The Creole community in Southern Louisiana is OLD. It dates back to Spanish rule and is and was tremendously independent-minded and somewhat insular. Prior to the U.S. Civil War, the "Free People of Color" community in and around New Orleans maintained a much higher level of wealth, education, and cultural sophistication compared to black slaves elsewhere in the South. Many had ties to Haiti and other areas of the French- or Spanish-speaking Caribbean.

During the Slavery and Jim Crow eras, lighter-colored Mulattoes could, and did, "pass" as white (the so-called "paper bag" test). This allowed them to marry whites despite local anti-miscegenation laws.

Due to flukes of genetics, you might have a scion of a very old and powerful Creole family with obvious Negro features who self-identifies as "White", or someone who identifies as African-American who could easily pass as a Northern European.

As for a mansion in New Orleans, it might have survived in an otherwise developed area because of its historical value. Especially after about 1920, there was a push to keep architecturally significant buildings from being destroyed. After 1966, the property might be listed as a National Historic Site, which provides tax break in exchange for severe limitations on what can be done with it. Even routine repairs have to follow certain rules so that the building remains substantially unaltered from its historic appearance. That makes such properties much less valuable commercially.

If you want a reason why the property's value is depressed, in addition to all the hassle associated with owning a historical building, it might require modern upgrades - anything from plumbing to hurricane-proofing - which are incredibly expensive to do with historically-acceptable materials.

Additionally, ever since the end of WW2, the entire New Orleans area has been bedeviled by the Formosan termite which attacks wooden structures by preference. The mansion could be literally crumbling due to bugs in the walls, with repairs and extermination potentially costing millions.

Another possibility is sinkholes. Due to geography and overuse of ground water, Southern Louisiana is filled with sinkholes which threaten nearby structures. Even a suspected sinkhole makes commercial developers run the other way.

Combine the three and you get gorgeous building that looks like Tara from "Gone With the Wind," associated with one or more gristly but important events in the nation's past, plus untold personal tragedies, which is a literal money pit due to termite infestation, foundation subsidence, and the need for serious structural upgrades. Meanwhile, the spacious grounds, with their magnolias and cypresses covered with Spanish moss, are subsiding due to a relatively recent sinkhole.

At least officially, the mansion and its grounds are a basket case, but that just makes it easier for the monster hunters to keep a low profile. Given its history and troubles, it could be the source of several adventures, especially if the termites aren't normal termites, the sinkhole leads to an actual cave network, and repairs to the walls and foundation reveal evidence of supernatural horror.

If you want a bit more isolation, move the mansion and grounds to someplace along the shores of Lake Ponchartrain. Still a relatively short drive to downtown NoLa but with a bit more space on the grounds for clandestine activities.