While bitcoins can't be blocked, the terrorist still need to convert their coins. Either directly use bitcoins for doing purchases or to exchange it into fiat and then use them for illegal purposes. Both of these ways can easily be tracked.
We can call that terrorist is stupid and doesn't have enough knowledge about privacy concern.
But they can track it no? Though the effectiveness is still questionable since mixers are a thing, crypto assets could still plainly be seen as to where they would go, and the assets would still need to be converted to fiat since most businesses don't even know what Bitcoin is, let alone accept them. Besides in this case, the opposite could be said as well right? Having too much control over the assets of a country, being able to block them just by presenting the public a "reasonable" excuse, then they could freeze assets of various powers.
It almost impossible to track the coins belongs to after it already
cleaned with a mixer if you use >1 inputs and send to multiple address, because no one know which address you got your coins from.
To answer this question, it depends on how you view about Bitcoin is.
Does Bitcoin has a value? Yes
Does Bitcoin solve centralized control (e.g. freeze funds, slow transaction)? Yes
Does Bitcoin can be used for charity program? Yes
Does Bitcoin help scammer and fraudster? Yes
It's
double edged sword and there's no end of this discussion since both of them is true.