Is that a rhetorical question? Cause btc itself is a investment, you don't need to invest it. You don't buy gold and invest, then ask where to invest gold. Bitcoin don't need further investment, while you could buy and sell weed if you are in USA states. Maybe open a business with it.
Maybe OP was asking, if you had Bitcoin (and want to use it elsewhere), where would you invest it? Like would you buy stuff from the darknet using your bitcoins and sell them off here in the clearnet? Or invest in ICOs or new altcoins. Something like that.
Yes i belive this is what op asking.But buying something on blacknet and sell it at clear net is not recommended because like we already know,most of the thing we buy in darknet will give us a problem in future.