Bluesphere gives the fog haze etc a blueish tone, as microsoft left this out..
If you google it 'FS2004 Bluesphere' there are some pics on a forum of before and after you'll see.. ;)
Yes you can us FSSkyworld and Bluesphere i think, but dont install the clouds or anything other than the dawn and dusk.. as this will remove bluesphere..

But imho, you dont need it when you have Bluesphere..