Stay away from the 2004 to lets say 2008 Colorados.
As mentioned, they had head issues as well as electrical gremlins that go unsolved. I believe in 08 or 09 they went from the 3.5 liter I5 to a 3.7 liter I5 and solved the head issue(I think).
I have a 2010 4x4 Colorado with 97k miles and i really like it though i never tow with it. I also have a 2003 4x4 S-10 with 242K miles (both trucks I bought brand new)so i can tell you the difference you will feel between the 4.3 you have in your blazer and the 3.7 I5 in the Colorado. The Colorado accelerates A LOT faster than the s-10 as it has 220hp vs the 180hp in the s-10. HOWEVER, and I cant remember the exact number, but both engines have roughly the same torque. The 4.3 feels like it has more pulling power, whereas the I5 engine feels, "zingy" for a lack of a better word. It feels like a hopped up 4 cylinder. That being said, it should pull your golf cart around with no problems.
I get around 19-20 mpg, 22 on the highway.
The only problem I have had with my Colorado is the instrument gauges. The tach went out at 45K miles and the temp gauge is wonky. So I bought a Scangauge($120ish vs $800ish for a new instrument cluster) that I mounted under the dash so I can watch the temps.
There are other subtle differences i can talk about, but thats the biggies.