I think the resulting finish depends on the type of sand/media used. I've used regular silica sand on several manifolds and ended up with a dull gray finish. Something a bit smoother (like bead blasting) would probably give a somewhat shiny silver gray finish like veno described.
In any case, freshly blasted cast iron won't stay clean and new looking for long. You'll need to apply some high temp paint or other protective coating so they don't get all rusty again.
I have had the best luck doing the following: media blast with medium abrasive...clean with solvent and blow dry . Then shot peen them at a machine shop....this is small metal shot that is used on engine blocks and heads. It smooths out the finish and removes the rougness from the sand blast and cast iron. Clean again....prime with high temp 1200* grey primer....finish with POR 15 gray high heat manifold paint. It is also a good idea to check the stud holes and run a tap thru them to clean up the threads....then install some new exhaust manifold studs and you are ready to go. You can also re-finish the heat riser the same way unless you are installing a new one.