It won't delete the bluecardfix and it will reset your mset so you'll have to re-use whatever installer.nds with a flashcart to reactivate the exploit (other than that, if it's a downgraded mset it won't upgrade it back).
Those cias are part of the firmware now and they won't get deleted by a simple formatting.
N3DS or O3DS or 2DS? If N3DS, no, it will brick when format system memory with downgraded MSET. If format system settings on downgraded MSET 4.x on 2DS, it also bricks the 2DS.
N3DS or O3DS or 2DS? If N3DS, no, it will brick when format system memory with downgraded MSET. If format system settings on downgraded MSET 4.x on 2DS, it also bricks the 2DS.
The problem with 2DS is that when you start it up after formatting it will ask you to turn on the 3D slider, and won't continue until you do. So since you don't have a slider you can't get past the setup - "softbrick".
This is because when 4.x was around, the 2DS did not exist. In 6.x the 2DS did exist so it contains the special different startup sequence for the 2DS that doesn't make you adjust the slider.