will a Blazer 4 x 4 trans swap with a 4 x 4 Silverado trans?

There will be no problems I am aware of if it is the full size K5 Blazer. The K5 Blazer is built on the C/K pickup chassis. The smaller S-10 Blazer though is built on the S-10 pickup chassis.

*The labor cost to change the transmission will depend on the hourly labor rate and whether or not the job includes removal of the donor vehicle transmission. Labor hours to remove the transmission and install the replacement is about 6 hours. Estimating the average hourly labor rate of an independent shop to be about $65, you can expect the labor cost to be $400ish. Add to that the cost of about 15 quarts of transmission fluid (about $3.00 per quart for generic brand) and the cost of the front and rear transmission seal (about $10 each). It only takes about 10 minutes to replace those seals while the transmission is out and so it just doesn't make sense not to change them when changing the transmission. And if you have to remove the donor vehicle transmission you can expect to pay about 2.5 hours additional labor.

Yep, they will swap. Both have the 4L60 (aka 700R4) transmission.

Since both are a 4wd unit, it will be a direct swap.