Most smokeless powders are uniform in shape within any given powder number or name, ie: Bullseye, H4227, IMR 4350, AA 2230, VV133, etc - and as The Mad Monk, says, they will not dissolve or break down in water.
Smokeless shapes are disks, disks with holes, disks or different colour, sticks- ie: tubular usually with a tiny hole through the middle, stcks of different colour and length from very shot to quite long, also little round (spherical) and/or flattened balls called ball powder or spherical powder.
Black powder is all different shapes and sizes within reason within each sample. It is angular, never evenly or similarly shaped, unless you call irregular, evenly shaped.
There is no comparrison, really, they are so visually different.
Using smokeless powder in a muzzleloader can be very injurious to you, your gun and also the people around you who will be quite happy, if they survive the occurrence, to sue you into poverty.
Yes - I know, Savage Arms built an in-line bolt action muzzleloader they sold as being used with smokeless powder. Seems to me, the load was also a seemingly small charge of large pistol/small rifle powder they suggested, Hodgdon's H110, I think it was. I seem to recall a few of them blowing up through the use of an inappropriate smokeless powder, or amount of smokeless powder.
Some people do not know the difference between a weighed and stricken measure & telling them will not do any good.
See - even an in-line, bolt action ML designed to be used with smokeless can blow up if someone stupid loads it.