Question: We were on holiday and had no shofar for shacharis. We only managed to obtain one later in the day. Should we have blown it at maariv?
Answer: The Rema (OC 581:1) writes that the minhag Ashkenaz is to blow the shofar each morning after shacharis throughout the month of Elul. He notes that some have the practice to blow in maariv, too. The Mateh Ephraim (Elef Lemateh 581:8) quotes the Elya Rabba who records that this was the practice in Prague.
However, the Aruch Hashulchan (OC 581:12) writes that one must not blow the shofar at night-time even to practise as that is not an opportune moment for mercy. Therefore, R’ Moshe Feinstein (Igros Moshe OC 4:21:5) explains that the Rema is evidently referring to those who davened maariv while it was still daytime. He notes that the Chayei Adam (138:1) specifies that some people would blow a second time during mincha which is still an acceptable time.
In conclusion, if a shul did not blow the shofar during shacharis, they should blow it later in the day. However, they should not blow the shofar at night.