tl;dr / sums it up at bottom
You're right not to move the f pawn lightly, but this is just one of those positions where opening the a2-g8 diagonal isn't bad. A few points:
- c4 blocks white from using it easily
- Be6 can be played right away to control it
- White has no light square bishop
Here's an opening example
As for blocking, it's true f5 blocks the bishop. In this sort of structure Bf5 is a good move to pressure the backward d3 pawn on a half open file. However in this specific position the e4 knight is in the way, and white is in solid control of e4 so a bishop on f5 just isn't doing anything.. Also moves like Bg4 and Bh3 don't attack any weak points / pawns (also neither is safe right now) so black isn't unhappy to give up those potential posts.
Black will try to pressure the d pawn with moves like Be6 (this also has the benefit of keeping the knight out of d5) Qd7 and Rad8. This is a situation where, at least for now, the knight is actually better on c6 where it helps prevent white from playing d4 to eliminate his backward pawn. By playing Nd4 you block the file and the knight no longer controls d4 (pieces don't control the square they sit on 
)
Ok so f5 is safe, and doesn't block our planned development (moves like Be6 and Qd7) but it's also not necessary for that d file presure, so even though f5 isn't bad can we say anything good about it? Yes. It chases the white knight out of the center, and f5+e5 controls a nice chunk of territory together (g4,f4,e4,d4). So first f5 with tempo, then when the d file pressure wins the pawn it will be better than if we'd won it without having played f5.
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tl;dr
No, I wouldn't worry about this. Nd4 is a central outpost, so it looks like a natural move. and f5 will often be a bad move for the reasons you said. In this specific position though, f5 is not weakening and the main theme is the backward pawn on the half open file (d3).
This is a position from a game I played yesterday and I am reviewing back today. I was black and it was my turn to move.
I played Nd4 however the computer said this wasn't the best move, not even top 3.
f5 was the recommendation but I am struggling to see why. It weakens the king's defence leaving a gaping diagonal for white to exploit with a move like Qb3. Also f5 blocks my bishop from the same f5 square and g4. What are the thoughts behind this move and should I be concerned that I didn't find it?