First, from the Baltimore Catechism:
Q. 1241. What is blasphemy, and what are profane words?
A. Blasphemy is any word or action intended as an insult to God. To say He is cruel or find fault with His works is blasphemy. It is a much greater sin than cursing or taking God’s name in vain. Profane words mean here bad, irreverent or irreligious words.
Now, here’s what the Catechism of the Catholic Church says, first from the Glossary and then the referenced section:
BLASPHEMY: Speech, thought, or action involving contempt for God or the Church, or persons or things dedicated to God. Blasphemy is directly opposed to the second commandment (2148).
2148 Blasphemy is directly opposed to the second commandment. It consists in uttering against God - inwardly or outwardly - words of hatred, reproach, or defiance; in speaking ill of God; in failing in respect toward him in one’s speech; in misusing God’s name. St. James condemns those “who blaspheme that honorable name [of Jesus] by which you are called.” The prohibition of blasphemy extends to language against Christ’s Church, the saints, and sacred things. It is also blasphemous to make use of God’s name to cover up criminal practices, to reduce peoples to servitude, to torture persons or put them to death. The misuse of God’s name to commit a crime can provoke others to repudiate religion.
Blasphemy is contrary to the respect due God and his holy name. It is in itself a grave sin.
Hope that helps!