I have the original copy of my grandmother's blessing certificate. I was wondering if I could added it to her memories page as a document. I didn't know if the information on the certificate would be sacred.
You can definitely add it. The information listed on a blessing certificate is really no different than that on any baby christening/baptism record. If you are at all worried, you can mark it as Private and no one will be able to see it.
In my personal opinion I don't think it would be a problem. However, all uploads are moderated and if it is a problem you will be notified.