ailita

Thank you so much for all the detailed answers. In the studio I go to or the workshops I followed, we were never told about the hazard of dry clay, oxides etc. I'm planning on setting a small studio at my place, whether it's for me or the others I'd rather be too cautious than not enough. At least I know now that I have to use a respiratory mask when handling powders. I'll try to use stains as much as possible since they are less harmful.

Thank you so much for all the detailed answers. In the studio I go to or the workshops I followed, we were never told about the hazard of dry clay, oxides etc. I'm planning on setting a small studio at my place, whether it's for me or the others I'd rather be too cautious than not enough. At least I know now that I have to use a respiratory mask when handling powders. I'll try to use stains as much as possible since they are less harmful.

I would say in general colouring the clay and glazing with a transparent gloss is going to be the safer option for the end user but there's so many variables in pottery that there's no one size fits all answer. 
 
https://digitalfire.com/picture/L9hivqD172
On this page Tony says "Stains are inherently much safer to use than raw metal oxide colorants because they are sintered as colorant/stabilizer blends. And much less is needed." So it is possible to make a black/coloured glaze that's food safe but it requires testing.

Engobe covered with a liner glaze might be another safer option.
I'd thought Hansen's reference to stains and safety had to do with safety for the potter in the process, but on that page he did line the ware with a black glaze*.
The colorants are the same thing once all melted, whether supplied via stain or raw material, is that true?
The stain is safer to handle.
The potter's safety, that's important too!
Hansen typically (what I can see) liner glazes and makes an argument for it, Liner Glaze (digitalfire.com)
*black glaze, for which he makes a case, in keeping with "Liner glazes can also be colored (if they are well tested and demonstrably non-leaching)."

So there’s a few points here I think. First, safety, and next practicality. All of the below assumes you have your heart set on a black and white marbling combo at cone 6 (1200 C) in an electric kiln. 
Safety:Most of the risks involved in pottery are to the potter, not the end user. Your exposure to any of those materials is going to be greater than anyone who’s using the end product, because the materials aren’t bonded to anything the way they are in fired ceramic.
That’s not to say that there are no risks, or that you don’t have to mind your chemistry so you have a stable glaze. But your safety is the most at risk in this scenario. Worry a little about your customer, but worry more about your own health.
Because the example is Manganese, most of that oxide’s dangers come from inhalation. Humans only absorb about 5% of the manganese they swallow, and it’s not absorbed through the skin. (Source cited here). So if you are not working next to a kiln full of manganese glazed wares while it’s firing, and you’re using good housekeeping practices that reduce airborne dry clay that uses manganese as a colourant, your chances of injury are lowered. That said, if you’re firing in reduction, using iron in any form as a colourant would be a much safer option if you don’t mind a very dark brown/warm black colour (disclaimer to come). If you’re firing in oxidation, a stain is safer than manganese.
Stains are designed to be more stable/less harmful than certain colouring oxides, and they create colours that are harder to get with oxides alone. They’re not soluble in their unfired form, so you don’t absorb them through your skin or anything like that. Some still contain spinel, so breathing the dust is still bad for your lungs, but only in the same way dry clay is. 
From a practical standpoint, If you’re making marbled clay by adding a colourant to a white clay body, this is the best method to ensure pieces don’t crack apart along the marbling lines. So what are the pros and cons of manganese, iron and black stain?
Iron and manganese are both pretty inexpensive. Iron is a lot safer to use for the potter than manganese, but doesn’t give a true black by itself. Both however, will off gas a lot more than a stain will. Black clay has been very popular the last few years, but all of them that use manganese or umber are prone to bloating if fired to cone 6 (roughly 1200 C). Manganese dioxide begins to break down into manganese oxide at 1080 C. This turns it into a more active flux, leading to bloating. The release of extra oxygen bubbles all creates a lot of pinholing or glaze blistering that is pretty difficult to resolve in an opaque glaze, and will make a clear glaze cloudy with micro bubbles. The few clays that I’ve seen that use iron or iron containing minerals don’t bloat any more than a normal red clay body would if overfired. But you’ll still have issues if you’re using a clear glaze because of that clouding. You can get a clear glaze to go *mostly* clear over an iron bearing clay, but it’s a lot of work and testing.
If you use a black mason stain in particular, they’re pretty refractory and don’t have any Loss On Ignition values, so that means they’re not loosing gasses. Solves all of those above problems, but they do cost a bit more.