I am not understanding a concrete block 1/2 wall.

Using an existing slab all depends on how high the wall will be and what use it will put to.

No one can tell you what" something like that " will cost but a local Contractor who has been informed. J.

A wall about three cement/concrete blocks high (appx 24") that will be stucco to match the house. Attached on top of the wall will be aluminum framing for screening. It's an outdoor screened patio and will still be used as such. I don't like the look of the manufactured aluminum kickplate florida patios are famous for and they are implying my patio will not support the weight of the concrete blocks. The slab is 4" thick. I absolutely do not want a wood framed wall. Do I need to ask a contractor or a mason?
Thank you

You need to ask a Contactor. J.

A 4 inch thick slab is not going to support much weight without cracking.