make new custom block available only if a specified mod is loaded

yes, this is possible, but you would need some coding knowledge.

First, lock the code of your item mod element(using the lock button on the sidebar) and go to its .java file. (using the pencil and lines button).

Then, find the method called "initElements". It should be around line 28. It would look something like this:

	@Override
	public void initElements() {
		elements.items.add(() -> new ItemCustom());
	}

Then, replace the above code with this and run control+w to add all imports.

    @Override
    public void initElements() {
        if(ModList.get().isLoaded("modid of the mod you want")){
            elements.items.add(() -> new ItemCustom());
        }
    }

Essentially what this does is check if a mod is loaded, if so, register this item in the mod element list. This would prevent the mod element being loaded if the mod isn't detected, which will further prevent the item being registered if the mod isn't loaded.

Wow, thanks, I was really looking for this, but would you happen to know how to register an item on a tab in another mod's creative?

I don't really know, sorry :(