Consequently, inflows remain high as investors continue to search for income. Fund flows for November were $2.1 billion, bringing year-to-date total to $10.4 billion. So far this year, MUB attracted $1.6 billion in net inflows, TFI brought in $385.9 million and ITM added $410.8 million, according to ETF.com.

Muni bond ETFs offer attractive tax-free income. MUB shows a 1.62% 30-day SEC yield or 2.86% taxable equivalent yield for those in the highest income bracket. TFI has a 1.92% 30-day SEC yile dor 3.39% taxable equivalent yield. ITM comes with a 2.03% 30-day SEC yield or a 3.37% taxable equivalent yield.

Looking ahead, BlackRock argues that municipal debt will continue to attract broad investment interest due to its unique characteristics, like tax-free income and attractive risk-adjusted returns, which should also help the asset category outperform. [How Much Can Muni Bond ETFs Grow?]

For more information on the munis market, visit our municipal bonds category.

Max Chen contributed to this article.