A dApp is a frontend + smart contracts [1]
HTML + CSS + js + jQuery should give you the tools you need for frontend.
As far as writing smart contracts, since Hive is a fork of Steem, I believe that tools like harpagon's steem smart contracts tool 2 can either be adapted or used directly to implement smart contracts with your existing javascript knowledge.
In terms of extending this knowledge to other blockchains, it's almost certainly worth learning some C++. This would give you an insight into Solidity [3] as well as a better understanding of the way blockchain developers think about building smart contracts into their products.
RE: Quello Question Thread #1