Diming halogens reduces the bulb wall temperature, and below a cut-off temperature (which I can't remember, but I have in a book somewhere), the halogen cycle stops. However, it also reduces the filament temperature, which significantly reduces filament evaporation in the first place. To get bulb blackening, you have to find a bulb with a geometry and power rating such that the bulb is below the halogen cycle temperature whilst the filamant still has significant evaporation.
In early halogens, there was a third factor - the max pinch seal temperature, above which the seal would fail where the conductors were passed through it. Designing a bulb to operate above the halogen cycle temperature but below the max pinch seal temperature in varying installation conditions of cooling (or not) gave quite a small operating window for correct operation of a bulb, and dimming it would often take it outside this.
I believe improvements in materials used mean that there's a much larger operating window for current halogen products, and the chance of going into an operating region where blackening can happen is significantly reduced.