Blending Before or After Roasting
The case for roasting coffees individually is strong with the Melange type blend (see below) and with a handful of particular coffees, such as Robusta on Monsooned coffees in espresso blends. Some coffees are denser, or have extreme size variations; these will roast differently than standard wet-processed arabicas. All dry-processed arabicas require roasting to a slightly higher temperature. If you have an established blend it certainly is easier to blend the coffee green and roast it together. If you are experimenting with blend ingredients and percentages you will want to pre-roast each separately so you can experiment with variations.
But in most cases the coffees can be roasted together and I would advise this: roast the coffee together until you encounter a situation where the results are disappointing and for success you must roast them separately. Every coffee roasts a bit differently but there is a great deal of averaging that occurs between coffees in the roast chamber, especially in drum roast systems. And then there are the coffees that do not roast evenly as single origins either: Yemeni, Ethiopian DP coffees, etc. Uneven roast color is not a defect, and only when it occurs in a wet-processed arabica that should roast to an even color (and sometimes not even in this case) is it of any consequence.