Theres a black screen for a long time before the "MOJANG" Screen!
This is normal, be patient. The mods are just initialising.
Why is grass growing on my lawn, I just cut it?
If you don't want grass to grow on your freshly cut lawn. Craft some "Weed Ex" from the hungryanimals mod (A purple and white spray canistor looking like thing) and right click on the ground. The sprayed grass should look kind of dry. It's done! No more pesky long grass.