I remember some guy post a review (more like a shill) about this exchange a few days/months ago. There is no serious accusation yet, but DWYOR.
From what I can understand, I need to hold both chains synced up, before I can start trading on their platform, and it seems like LTC/BLOCK is the most traded trading pair, from what I could see.
Yeah, you need to install a wallet and load it up before you can use it because they're non-custodial. Check their guide
[1], it looks detailed and easy to follow.
[1] https://docs.blocknet.co/blockdx/setup/