While we cover most seeds when we plant them, some need light to germinate, and others must have darkness. Normally, a seed needing darkness would have to be covered by soil, which can happen any number of ways. The instructions want you to sow the seed in flats or pots and ensure darkness by placing them inside a black plastic bag, right? That doesn't mean you can't look inside the bag to check on germination. The instructions are just giving you a an edge in helping germination.

Of course you can check! But don't be tempted to peek before twenty or more days. If you are using a heat mat to provide bottom heat, the seeds will germinate much more quickly.

Verbena seeds can take forever to germinate, weeks and weeks. I've actually never seen a black plastic recommendation before. Usually, a covering of fluffy germination mix is all that is required for seeds that require darkness and that's all I've ever done as a professional or a retired hobbyist.

I'd think that covering with black plastic would encourage mold and mildew and set the seedlings up for rampant damping off diseases after germination.

Anyway, I'm not going to confuse you by telling you how I would do it, but just ease your mind about checking for germination. Be very careful that the medium is not overly moist when you cover it or your project will not be successful.

I always used a piece of newspaper. Plastic seemed to make my soil too wet.

I always just covered seeds that need darkness with soil - the soil provides the darkness. What do you think happens outside in nature?

Mxk3, some seeds are so tiny that they wouldn't germinate under soil. Outdoors they might have a leaf over them or something to block the light, plus one seed out of hundreds might make it to germination. Indoors we have limited seeds and conditions so we have to use other methods.