I highly recommend buying and reading the D&D D20/3.5 version of Dave Arneson's Blackmoor. It's a campaign setting book, but it has much of the lore and information you might need to run a sandbox campaign. I was able to buy a brand new hard copy within the past year, but you can also get a PDF version through .
I would read chapters 1 and 4-6 for the core lore and information about the setting. There's good information on NPCs in chapter 7, although a number of NPCs from the original Blackmoor campaign are not included. Chapter 8 covers monsters unique to Blackmoor, although you can convert these fairly easily to 5E with WotC's official . There's also an adventure included in Chapter 9, and you can find adventure ideas and random encounter tables in the Appendix.
In terms of a starting location, the Kingdom of Blackmoor is an obvious choice since it's home to the city of Blackmoor, and the towns and areas have plenty of lore and information. The whole northeast region of the kingdom is ideal, including places such as the City of Maus, Jackport, Archlis, and Newgate. As you get into the Great Dismal Swamp to the west, between Blackmoor and The Duchy of Ten, you have plenty of additional towns and locations to work with. Then you have the duchy itself, which is occupied by the Afridhi and now part of their empire. Those are all prime locations for starting a sandbox campaign.
The big threats are the Afridhi from the west, the Skandaharians from the north, the Thonian Empire to the east and southeast, and the Egg of Coot and his mysterious, sinister workings in the Kingdom of Blackmoor and beyond. This is in addition to political and military intrigues involving the Duchy of Ten, the dwarves of the Regent of the Mines, Ringlo Hall and the Cumasti elves, the Westryn elves to the west, the various Blackmoor baronies, and the Wizard's Cabal. Then you have the fact that Blackmoor is a points of light setting with a lot of wild, dangerous and unexplored areas.
The D20/3.5 edition map is a great resource, although the you can find online isn't large enough and doesn't have high enough resolution to be really useful. Havard has a nice hosted on his site, but I recommend using Adobe's snapshot tool to create a digital copy of the large, hi-res map that comes with the D20-era PDF.
I have also been building a glossography reference for Blackmoor, based on the D20 book as well as a couple of the older sources such as the DA1 Adventures in Blackmoor module from TSR and even some snippets from the First Fantasy Campaign (FFC). It's based on all the locations you can find on the D20 Blackmoor map, but it's a work in progress and far from complete.
If you end up using the D20 book as a resource, I came up with some of my own playable race/class conversions using 5E options:
Docrae = Lightfoot Halfling Halfling = Stout Halfling Cumasti Elf = High Elf Westryn Elf = Wood Elf
Arcane Warrior = Eldritch Knight Wokan = Druid Monk = Order of the Mystic or Order of the Star Skandaharian = Barbarian Beastman = Orc or Half-Orc (Barbarian or Barbarian/Druid) Ash Goblin = Goblin Baleborne Orc = Hobgoblin
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