change border color focus
state: {
isFocused: true
}
handleFocus = () => this.setState({isFocused: true})
handleBlur = () => this.setState({isFocused: false})
<TextInput
onFocus={this.handleFocus}
onBlur={this.handleBlur}
style={[//Your Styles, {
borderBottomColor: this.state.isFocused
? 'black'
: 'red',
borderBottomWidth: 1,
}]}
/>