Is there a particular reason you need to sow indoors? Both are hardy annuals and would be happier sown in situ. Poppies (if these are either Papaver rhoes - based varietes or P somniferum) get fleshy roots which do not take kindly to transplanting. Nigella too is murder to pull up because of deep tough roots. Anyway, indoors or out I'd sow them now. A light frost won't faze either of them. Once you have them they will probably self sow.