prog question

Does anyone know what a blockchain actually is.

Observe carefully for I will not repeat myself:

. blockchain:-+        
.             |        
.             |
.             v                  
.        +-------------+
.        |    car: cdr:|
.        |     |    |  |
.        |     |    |  |
.        +-----|----|--+
.              |    |
.          ^   |    |
.          |   v    v
. bb: -----+               
.              2    4      
.                          
.              ^    ^      
.              |    |      
.              |    |      
.        the-car:  the-cdr:

  progpoaster       .-,(  ),-.    
   __  _         .-(          )-.       shitpost gateway  /prog/ server in North Macedonia
  [__]|=|  ---->(    internet    )-------> __________ ------> ____   __ 
  /::/|_|        '-(          ).-'        [_...__...°]       |    | |==|
                     '-.( ).-'                               |____| |  |
                                                             /::::/ |__|

some sort of decentralized database kept legit through cryptographic wizardry

keeping things legit and consistent isn't a blockchain innovation. it just uses hashing, i.e. for every block, a hash is generated, and if you change the block's data, you get a different hash too (there's a very small chance that you get the same hash, that's a collision, but assuming nobody has broken the hash function you can't forge such a collision).
Hashing as a form of signing data so nobody can tamper with it has been around for years.
The blockchain is a way to make that distributed so that there's one place where multiple people can add data so that nobody can tamper with it. Adding a hashed block to the blockchain is easy, and with multiple parties interested in the blockchain, and no one trusting anyone, you first need to come to a consensus who gets a turn to add data. The blockchain approach is to let everyone do a non-trivial computation, and the first one to solve it, can add the next block (the block they add should however contain multiple pieces of data, so in the case of cryptocurrencies, multiple transactions, not just their own). This process repeats, and next time someone else gets to add the next block.
From a technical perspective, this process of deciding who gets to add a block is the blockchain "innovation". You decide if you consider that a revolutionary game changer.