BlockChain basically provide Cryptocurrency(like bitcoin) wallet service.first of all if you do not know about What is Cryptocurrency then check it on google then come to blockchain.
So basically blockchain provide a storage area for your cryptocurrency, They are very popular and 100% legit+ trusted.
1-Blockchain is on top because of their good service and account security.Blockchain currently provide storage for 2 cryptocurrency which are BITCOIN and Etherium.
2-Now you can also buy or sell your bitcoin through blockchain because blockchain is tie up with Unocoin.
3-You can also exchange your bitcoin to etherium or vice versa on blockchain.
but still there is some lacks in blockchain which is it only support two cryptocurrency currently(BTC and ETH), means if you bought or mined other currency like Monero or Dash then you can not store it directly on blockchain.
I said not directly but indirectly you can by converting your other crytpocurrency in Bitcoin or Etherium.
So overall it is best wallet in market on the basis of security, privacy, trust and service.