There is good FAQ here on BER, causes, and suggested treatments. Just click on FAQ link at the top of the page and scroll down to Blossom end rot. In this case, I agree strongly with the others your container is much too small and your plant is likely rootbound and exceptionally stressed. It will be quite difficult to prevent BER in this case. Try removing a sucker from it, rooting it in a glass of water and then planting it in a much larger container. There are several posts here on how to root suckers. Good luck. Dave

ML, For Aloes, it's easy. Healthy Aloe roots are yellow (the shading depends on the species of the Aloe and the age of the root). It is a natural process for older, thinner roots to die off, so when you repot go ahead and clean up the root ball. The old ones look dead and peel off pretty easily.

No Maria, you've got to cut back more until you have good green viable stem. And as Jeff (CactusMcHarris) has said you don't need anything on the cut end. I leave cutting in warm indirect light sometimes for weeks and they just root from the natural humidity in the air. Having said this, it might not happen in very low humidity areas, but here in Michigan we almost always have 40% ~ 50% RH. I just cut this from one of my Echeveria to demonstrate what you want to see after cutting. I would allow my cutting to callous for at least a week until I see the cut end dry and looking like a callous on your hand or foot; it will look hard and dry and somewhat rough. The mix is something I make but you don't have to have the same thing. Again, as Jeff has said, Perlite is readily available at any Big Box store or Garden Center. Do NOT use regular sand because it packs down too much and dries slowly. If you have a problem finding something course or gritty, go to a pet store/Walmart/Target and ask for bird gravel; the stuff used on the bottom of a bird's cage that they eat to help digest food. It will be a bit pricey but if all you need is a small quantity, it will work. I'm personally not a fan of "Topsoil" as a portion of my mix because it won't dry quickly for me.

Root rot makes the roots soft and mushy.Give the roots a pinch and remove any that are soft.You will be able to tell easily.If you don't have a cold winter,then I would soak the entire plant in a bleach solution of 1 part bleach to 9 parts water for 1 hour to TRY to prevent rust.There are no guarantees. Betsy