Using `black_box` directly on the `.set` call is the same as writing

```rust
bitv.set(..., true);
black_box(());
```

i.e. it is not going to force the `set` call to stay.

Writing it as 
```rust
bh.iter(|| {
    bitv.set((r.next_u32() as uint) % uint::BITS, true);
    black_box(&bitv);
})
```
or returning the reference as above, should be fine.