Short Answer: The blaze is mostly dust proof. I cannot confirm how dust proof. The only things I would worry about would be really fine particles, perhaps particles made from cutting tile or something of that nature.
Long Answer: Devices are normally rated with an IP rating, testing for dust and water resistance. I've been looking for a while now, but I cannot find the IP rating for fitbits. I've read on some website that I don't fully trust that the Blaze is IP67. The first number represents the dust rating while the second represents water resistance.
6 - "No ingress of dust; complete protection against contact (dust tight). A vacuum must be applied. Test duration of up to 8 hours based on air flow."
However, I'll trust tap water from Kenya before I trust that sight. Common sense tells me a vacuum would not be possible for the altimeter. It probably has a rating of 5.
5 - "Ingress of dust is not entirely prevented, but it must not enter in sufficient quantity to interfere with the satisfactory operation of the equipment."
The Blaze is most likely closer to a 6 than a 5. But I cannot confirm.
