And yes, at least one exchange needs to offer short selling on ETH and be implemented in Blackbird to do BTC/ETH arbitrage.
Warning: if you do BTC/ETH arbitrage you will be hedged against the market risk on ETH but not on BTC anymore. USD/BTC and USD/ETH would be safer, unless you are willing to accept BTC risk.
Does it still need to short dealing with only cryptocurrency ?
Time for transfert a far lower between market when dealing with cryptocurrency only but still need one or more hours to confirm transaction.
But i understand it's totally out of bot's strategy to perform any transfer between markets.