下面是解决方案
创建了像这样的JS方法：

const blobToBase64 = async blob => {
    return new Promise((resolve, reject) => {
        const reader = new FileReader();
        reader.onload = () => resolve(reader.result)
        reader.error = (err) => reject(err)
        reader.readAsDataURL(blob)
    })
}

window.convertBlobURLToBase64 = async (url) => {
    const response = await fetch(url)
    const blob = await response.blob();
    const imageBase64 = await blobToBase64(blob)
    return imageBase64; 
}

字符串
使用JSInterop获取base64字符串

private async Task UploadImagesOnBlobAsync()
    {
        var result=await _jsRunTime.InvokeAsync<string>("convertBlobURLToBase64", _card.LogoImageDetails.ActualImage);
    }

型