There are many many posts about drilling or connecting to blind flanges - please search for them.
However the largest size you can attach to a 24" blind flange is 3 1/2" without using a hub flange (table 6 of ASME B 16.5)
The calculation will be to ASME VIII not B 31.3.
It probably won't pass without a LOT of reinforcement.
Far better to either weld a 24 x 16 reducer to a 24" WN flange and then a 16 x 8" reducer to that or use an 8" reducing hub flange or try and get a blind flange made with a hub of suitable thickness and then drill it out.
You unfortunately need to tell your client that they can't just weld on an 8" pipe to a standard 24" blind flange and be compliant with the design codes. Ignore words such as "temporary" or "commissioning". If it has pressure in it design it properly otherwise your risk to people and property increases and then it will be YOUR fault, not the clients....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
