5 week old chicks with iris color change;Is this "Marek's disease"

It's probably Mareks - the ocular variety - and, yes, they will likely be blind in one or both eyes. Blind in one eye is survivable, but a totally blind chicken is in a real bind because they have problems finding food/water, protecting themselves from other chickens as well as predators. Personally, I'd never impose a life of blindness on one of my chickens. I'd euthanize if totally blind. As to the other symptoms of Mareks (paralysis from brain lesions and tumors that effect vital organs) they are caused by separate strains of Mareks. Your birds may never suffer those other symptoms, but they could, so you should also take that into consideration as you are deciding what to do about your grey-eyed Silkies.

If you've been reading about Mareks, you also know that it is highly contagious and can infect any new unvaccinated birds you bring into your flock for more than a year in the future. Any other chickens you have now should be presumed infected. In light of the seriousness of this disease and what it means for you in keeping chickens into the future, I suggest you contact your State Veterinarian through the Department of Agriculture and send one of your birds for a necropsy. This is the only way you can confirm whether or not you have Mareks running through your flock.