So in your scenario
Buy 1 ETH for say $500, your cost base is $500.
If then trade the 1 ETH for bitcoin, it only depends on what ETH is worth at that moment.
If 1 ETH is now worth $400, then you have a $100 loss, if ETH is worth $500 then no taxable gain or loss, if 1 ETH is wroth $600 then you owe tax on $100 of capital gains.
What bitcoin is worth is irrelevant until you sell the bitcoin.
EDIT Just assume that every transaction between crypto currencies has an implicit, convert to USD first and then buy the other Crypto with USD.
Also, There is always a USD price, what the price is, is a bit of an art, but its what ever price you can convince the IRS of.