Dots do not overwrite regen, but do block if you are dot'd.
http://www.tski.co.jp/baldio/patch/20000719.html
Regen Effects:
This week we corrected a bug that caused regen-type spells (e.g. Regeneration, Chloroplast, Regrowth, etc.) to overwrite DOT spells. As a result of this fix, players can no longer accept a regeneration spell while under the effect of a DOT. However, if the regen spell is already in place, the DOT will stack, but not replace the regen spell. This will allow users of beneficial DOTs, such as Call of Bones, to continue using their DOT while under the affect of regen.