In my last game I wanted to set a charge in a plaza and move the squad inside a building. Then during the next activation of this squad, I wanted to detonate. However, my opponent insisted that they would get no cover from being inside of a building with a featureless wall between the squad and the charge since they where within range 1 and the card read blast to ignore cover. The only entrance and window to this building where outside of range 1. Was this a correct ruling?

No it wasn't.
RRG page 13 under "Area weapon":

"When using an area weapon, perform a separate attack against each unit that is in line of sight and at the range indicated by the number on the range icon."

It doesn't matter if the attack pool has blast or not, if that mine can't see any of the minis in the defender, it can't attack it.

And I’ll just add the entry for Blast:

An attack pool that includes a weapon that has the blast
keyword ignores the effects of cover.
• During the “Apply Dodge and Cover” step of an attack, the
defender cannot use light or heavy cover to cancel hit (󲉠)
results produced by an attack pool that contains a die that
was contributed by a weapon that has the blast keyword

In summary: Blast ignores cover. It does not ignore line of sight. So the unit inside the building is safe from the attack.

Thanks guys. I thought it could not wound my squad since there was no line of sight. Every hex and counter war game I have played has always come down to line of sight and a satchel charge isn't exactly artillery. I figured Legion was no different. Also, I noticed that I need to update my RRG book since I don't have that on page 13.