For a digital game to be provably fair it means you have data about the next thing you do BEFORE actually doing it so you can prove (AFTER playing) that what you did was decided before you actually made a bet and started the round. For better understanding let's make an example by supposing you play a simple dice game where you can roll a number between 1 and 10 (so you have 10 possible values you can get in total, which are 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10). It is considered to be provably fair if the site tells you in advance what the next roll will be (not necessarily in an understandable or decriptable format at the beginning). After you play then you can verify that the result corresponds to what the site said. For example they can give you the hashed value of the next roll but you won't be able to know what the next roll will be untill you make a bet and play (you can go from "roll" to "hash" but not from "hash" to "roll").



You can try and implement a script that makes your game provably fair, tho' I don't really know if that's possible at all for two main reasons:
1. I don't even know what the rules of black jack are;
2. Being a cards game I don't know if you can really write such a script since every time one card goes out it cannot go out again so your data will change every single time.