Most likely they are but it is impossible to say for sure without knowing what type of seeds. Assuming that black spots are NOT normally found on these seeds, then yes either they weren't dry enough and have developed mold, or they were infected to begin with from infected plants.

Saved seeds do best when first, collected only from non-diseased plants, fermented to remove any gel coating, disinfected by rinsing in a mild solution of chlorine bleach (10:1 water to bleach), then air dried thoroughly for several days or even weeks, then stored in paper rather than plastic. Plastic allows for retention of any residual moisture in the seeds while paper does not.

If you wish to try some of the seeds you can try whisking them lightly over sand paper to see if the mold comes off (works best with large seeds), then disinfecting them immediately before planting (as they will be wet again). Some will germinate. But I don't know of any way to tell how healthy the plant will be so you have to make the choice as to whether it is worth the time and effort vs. buying new seed.

Good luck.

Dave

Laurie,
Did you put them in plastic and black formed? If you did then you killed the seed. And if you did don't feel bad, we all make mistakes like that.
Once dry enough to store, I put my seeds in paper envelopes. Sometimes I make them and sometimes I use old seed pkts. depending on the quantity.
Oh and some seeds are naturally black like some pepper seed.
Remy