Whether de novo mutation can be inherited or not?
De novo mutations can be inherited if they occur in the germline cells, those responsible for making the sperm or the egg. If you have a de novo mutation in a blood cell or a skin cell, this doesn't pass on to the next generation. But when you test the blood for mutations, it can also mean that the same mutation is present in the germline! However, if the parents are not sufferers of the dominant disease, it means this mutation is not present in their somatic (non-germline) cells. It's likely that the mutation occurred de novo in either the parent's germline during their lifetime (during the production of their sex cells in adulthood).

If your parent has a mutation in their germline, and it is passed on to you, the zygote is formed from both your parents (half of your alleles are from the mutated parent) and this zygote creates ALL of you, including your blood cells and your skin cells. This is why we can test your blood to guess what must have been the case in your germline. Of course, you may have accumulated mutations independently in the germline in the meantime, or accumulated mutations independently in the blood cells! So it's not completely the case that testing blood will tell you about the state of the germline. But you must understand that ALL of you is created from a single cell, the zygote, so at this point there is a 'bottleneck' that forces all somatic and germline cells to share the same genetic makeup. 

Also, please consider that humans are diploid, i.e. have one copy from the mother and one copy from the father. And that these recombine and only one copy goes into the sex cell. This may help you think about your problem.

I trust this will help you better understand your questions.