From the name it seems that 2.5 is the answer. 2 and 1/2lbs per cubic foot of material. What is considered good quality memory foam is usually 5.0 or more.
If it was of quality it should not have gone flat. Once it has reached that stage, you probably will not be able to get it back to its original state....
Yes, you can use an electric blanket with a memory foam mattress topper. You might actually enjoy the feel of the mattress more when using an electric blanket because they often times feel different...
Most foam mattress toppers have a beige or white colored covering. The foam on the inside of the mattress topper may be any color, grey, white, blue etc.