Anytime you flash anything to your phone (whether it's a rom, kernel, mod, whatever) via any method (whether it's Odin, CWM, whatever) there is a very small risk that something will go wrong. Each and every time. This risk always remains small with every flash, it doesn't mean the 100th time you flash is 100 times more dangerous than the first time or more likely to damage your phone. If you attempt to flash with hardware that isn't working perfectly or you fail to learn/understand what you're doing before you actually do it, you increase that risk.
If you ensure you know exactly what you're doing before you do it (the reading/learning bit I alluded to in my PP), and ensure you don't do anything completely daft like attempt to flash firmware for a completely different device to your phone, or pull the cable halfway through an Odin flash, you ensure that risk remains very minimal.
However, if you're looking for a 100% guaranteed risk free way to mod your phone, there isn't one.
So, can you install Siyah safely to your phone ? Yes, you can. But as I've said above, there is always a small chance you might brick your phone in the process.
If it's important enough to cause you grief if it's lost, back it up. No excuses
Trolling for thanks is lame
Does Samsung's auto-brightness implementation crap drive you to self-harm ? A solution !
Failure to search is the leading cause of male impotence on XDA
If you don't know how to recover if things go wrong, what are you doing flashing custom roms ?