Docs for retrieve batch transactions says txHashes[]=a&txHashes[]=b&txHashes[]=c, but the example indicates an unbracketed version:
curl https://testnet.helloblock.io/v1/transactions?txHashes=6f9e9570881e781db8c137c84c111a138e4a022e6b2def5e2a1589a802fe25f3&txHashes=770e6e4c66fc61fb523e5aefe11780b26c8473638e7065ca726a6492ab7f6345&txHashes=47bf46f92384002dc008696dac3437a2ca4a2696c21a3f6d1d789513e7b9a3f0
API does not accepted the bracketed version, as I have discovered using Faraday.