How to make bitcoins from other cryptocurrencies?
The only way to convert them is by trading one for the other, or by going through a third currency. 

Meaning:


Find a person or exchange platform that trades exactly the currency pair you want e.g. LTC/BTC or BLK/BTC.
Trade LTC -> USD -> BTC


Some exchanges trade a number of different altcoins, so they would lend themselves to do what you want. I have no experience trading anything but Bitcoin, but I've seen that for example HitBTC trades LTC/BTC, and BTER trades BLK/BTC.

I have used neither exchange and therefore cannot recommend them, just giving them as examples.