First off you can go back and change them if you make a mistake. But to answer your question if you were playing on live servers, the shadowblade damage is 100% based on strength. But because this is classic your damage is split based on other stats like str/dex/quick. Sorry i cant give you an exact answer but quick also affect combat swing speed so 10 points there. Dex also affects evade so 10 points there (maybe less.) And assuming classic had a 3 way split between the three i mentioned above 10 points in strength for higher damage... or more
No SB damage is based on anything but Str. You cannot change your starting stats, it looks like they change, but it doesn't stick. With that in mind, you primary stats are going to be in the 300-350 range at 50, so a few points here or there is not a big deal.
All of the Shadowblade's damage types depend on strength, regardless of spec, because Midgard doesn't have a one handed piercing line, like the other realms do. That being said, you should probably put 15 points into strength. For the remaining 10, I would recommend Quickness, as it increases your swing speed. Dexterity is nearly worthless for you, and is the "Primary" stat raised by leveling up for the class, so you won't need any points into it. Constitution could be okay for a few extra hits, but I personally think Quickness is more important. As stated previously by an above poster though, there isn't a lot to worry about if you didn't min/max your stats exactly right, because the variance from roughly 10 points in a stat is not going to make or break you in the end.