Is it safe to pour concrete and use mortar in freezing weather?
Yes, you can pour concrete in freezing weather, but it’s complicated. 

First of all, you’re talking about “grouting” cinder blocks, not pouring concrete. 

There are many sites you can Google for “cold weather concreting”. I’d stay with a site that refers to ACI standards. Here’s one:

https://www.concrete.org/Portals/0/Files/PDF/University/306R-16_excerpt.pdf

Many of the procedures for cold weather concreting is the same as for mortar and grouting AND may require a “tent” over the work area so you can heat the work space. 

I would only attempt this with an experienced cold weather concrete contractor. I’d also review procedures with him so you both have a clear understanding of what is expected by the contractor if: 1) the weather turns colder than normal, 2) time delay maintenance of construction site waiting for weather to change, 3) costs for extreme cold weather delay, etc. 

I’d also define the lowest temperature the contractor can work in and who will monitor the site to verify compliance with: A) thawing of ground (if you’re pouring on ground), B) maintaining air temperature around work site during work and curing period, C) maintaining drum temperature, etc. 

As you can see, it is possible to work in cold weather, but it’s expensive AND risky. The contractor knows he’ll be working slower and doing less than working in warmer temperatures...so he’ll increase his cost to account for the risks. You may want to wait for better weather.