There is no such thing as a SAFE blank, especially when you have actors shooting at other actors. Always use EXTREME caution.
Brass and primers would be normal as sized and primed.
You will need a wad. We used a punch and hammer to cut out the wads. The wads were about 0.040 to 0.050 oversized i.e. for a .45 LC the wad diameter would be 0.50 inch. Our punches were custom made.
The wad material needs to disintegrate with the bang. Tide detergent or cereal boxes did work; also some pressed paper drink coaster (like you would get at a lounge). Find something that cuts cleanly, has enough tension to keep the gunpowder inside the brass and pressed against the primer until fired, and then disintegrates.
A blank with some flash and some noise can be made with small charge of fast smokeless powder say 5-7 gr of WW 231 or Bullseye. Size the case. prime, add the measured powder, use a wooden dowel to press the wad on top of the powder; and then you will probably want to add a roll crimp not the hold the powder but so you can feed the blank into the chamber.
A very load blank with lots of flash and bang can be made by adding Black Powder (FFF) on top of the smokeless. More realistic; and much more dangerous. A blank with black powder will easily blow a hole completely through the Yellow Pages of a large sized city. This can also seriously injure or kill someone close to the fired blank.
A blank used to signal or make noise is somewhat safe because it is fired into the air or a safe direction. Any blank used for theatrical or movie is DANGEROUS because it is aimed in the general direction of other people. Don't forget there is unburned powder, wad material that did not fully disintegrate, and secondary projectiles (dirt and stuff that is picked up by the blast). When we did our shows we did not actually aim at each but rather off to the side.