This isn't very blackhat but you might have something similar in the country you are located. Here we have something that in the US is called "Treasury Bills' (You can look at Investopedia) which you can invest in. Depending on where you are located you can invest it for 1 month, 3 months, 6 months or 1 year and you get a certain % at the end of that period. Most banks now have savings accounts that give you daily returns (very small) so you could keep the money in one of those accounts and let it build.
Those are the most secure way to ensure that you won't lose anything but the returns are a lot.
You can invest it in Bitcoin by buying and then selling when the value is higher than what you have bought it for. But the opposite can happen meaning that the value can actually go down.