I've been researching different ways to blacken brass and steel such as with blackening agent, electroplating, etc. I don't know which one I should try to achieve a finish like ones given in the picture, I also want it to get a matte appearance without having to add matte varnish.
A reverse image search suggests that the item of jewellery in that picture is plated in black gold; an alloy made of cobalt and gold. This might be ...difficult to do at home. Let's look at some alternatives.
This is probably not what you're looking for, but good quality matt black spray paint, sprayed over a decent metal primer coat, will be reasonably durable and very simple at achieve
You can either apply a coating of an oxidating substance, or oxidate the metal itself - a process known as bluing. This will give a relatively matt finish, but probably not as matt as you're looking for.
If you go for aluminium instead of steel/brass, then you can try anodising it. Anodising at home is tricky, and the cost of tools is probably more than it's worth if you only want to blacken one or two items, but it is possible.
It seems to be possible to get a matt finish when anodising at home
This is the boring option, but if you need something doing well and don't know how to do it, or don't have the necessary equipment, it's often possible to pay someone else to do it for you.
Here are some examples :
High quality: PVD titanium Nitride or, more shiny, diamond-Like carbon or black rhodium deposit. These are shiny on shiny metals but not on matte (I.E. sandblasted) metal.
Low quality: black oxide, black galvanic chromium or black galvanic nickel (but don't use nickel for skin contact)...
Aluminium only: black anodizing.
This cannot be done at home, well... galvanic treatments and anodizing could but they have some security concerns.
If at home you can use a high quality shiny paint or black glazing for ceramics.