You could potentially treat the black primer as your black, the potential issues have mostly been covered.
- If you have to touch it up, the touched up paint will look different (however this is alleviated if you apply a wash to the model when all is finished or if you apply a varnish at the end, also if you simply don't bother touching up your models it's less of a problem).
- You might actually want a very dark grey rather than a pure black.
- Sometimes when you spray, the deep crevices don't get enough paint in them.
For my Night Goblins rank and file models (not the characters), pictured here:
http://www.dakkadakka.com/gallery/images-59981-39860_Orcs.html I simply sprayed them with Tamiya flat black from a rattle can, I specifically chose Tamiya because it dries very thin and doesn't obscure detail, so I could lay down a few coats from different angles to make sure I wasn't missing any spots but also not losing detail.
From there I didn't bother repainting the black with a hairy brush, I just highlighted it with P3's greatcloak grey, then a medium grey, then gave a quick once over with a black wash.
They don't look awesome, they aren't going to win any competitions, but it was quick and looks good on the table (at least
IMO), if I'd spent more time I might never have finished the army.
The characters I used the same method, I just spend a bit more time trying to blend the grey highlights so they look better when you zoom in on them.
There's a lot of different ways of painting black and some look better on some models and others look better on different models.
If I were personally painting Black Templars, I'd actually prime them white then mask the areas that are going to be white (shoulder pads, knee pads, icons) before spraying them with a black spray, just to save me having to paint white over black.